French onion soup (soupe à l'oignon gratinée)

Name: French onion soup (soupe à l'oignon gratinée)
Prep time: 20 min
Cook time: 1 h 50 min
Total time: 2 h 15 min
Cuisine: French
Ingredients: 50 g unsalted butter; 1.3 kg yellow (brown) onions, about 6 large, halved and sliced 5 mm thick from root to tip; ½ tsp fine salt, plus more to taste; Water, for deglazing; 15 g plain (all-purpose) flour (about 2 tbsp); 150 ml dry white wine; 1.5 l beef stock, preferably homemade; 2 bay leaves; 4 thyme sprigs (or 1/2 tsp dried thyme); ½ tsp freshly ground black pepper; 2 tbsp cognac or other brandy (optional); 8 slices baguette, about 1.5 cm thick (day-old is ideal); 1 garlic clove, halved; 200 g Gruyère or Comté, coarsely grated

Onions cooked slowly in butter until deep brown and jammy, simmered in good beef stock, then sealed under toasted baguette and bubbling Gruyère.

By the Soupmap Culinary Team

Prep: 20 min
Cook: 1 hr 50 min
Total: 2 hr 15 min
Effort: Takes some care
Serves: 4

Broth richness 7/10

Spice 1/10

Comfort 9/10

Ratings by the Soupmap Culinary Team, not readers.

Photo: jeffreyw, CC BY 2.0, via Wikimedia Commons

The ingredient list is short, and the onions do nearly all the work. Cooked slowly in butter, a heaped pot of sliced onions collapses to a fraction of its size and turns a deep chestnut brown, sweet and savoury at once. That colour is the flavour of the soup, and it can't be hurried.

The mistake almost everyone makes is trusting a recipe that says onions caramelise in 15 minutes. They don't. Over high heat they scorch at the edges while staying pale and sharp in the middle. Give them 45 minutes to an hour over moderate heat, sometimes more, and keep scraping up the brown film that forms on the bottom of the pot with splashes of water. That film, the fond (French for "base"), is browned onion juice, and it belongs in the soup, not stuck to the pan.

The rest is good stock, a splash of dry white wine, and a lid of dry toast and cheese browned under a hot grill until it blisters. Gratinée means exactly that: finished with a browned crust.

Method

Caramelise the onions

Melt the butter in a large, heavy-based pot or casserole (at least 5 l) over medium heat until it foams. Add the onions and the 1/2 tsp salt and turn them until every slice is coated; the pot will be very full, but they shrink fast. Cover and cook for 15 minutes, stirring twice, until the onions have slumped, turned translucent and released plenty of liquid.
Uncover and turn the heat to medium-low. Cook, stirring every few minutes and scraping the bottom with a flat wooden spatula. For the first 15–20 minutes the liquid boils away and the onions stay pale; after that they start to turn golden and a brown film begins to build on the bottom of the pot.
From here, deglaze as you go. Whenever the film turns deep brown (before it looks black or smells sharp), add 2–3 tbsp water, scrape the film free and stir it back into the onions, then let the water cook off. Repeat every 5–10 minutes. The onions are ready when they are deep chestnut brown, soft and jammy, and smell sweet rather than raw, 45–60 minutes after uncovering, sometimes longer. If they catch faster than you can deglaze, lower the heat.

Build the soup

Sprinkle the flour over the onions and stir constantly over medium heat for 2 minutes, until it has disappeared into the onions and smells lightly toasted. It gives the broth a little body.
Pour in the wine and scrape the bottom of the pot one last time. Let it bubble until reduced by about half and no longer smelling sharply of alcohol, 2–3 minutes.
Add the stock, bay leaves, thyme and pepper. Bring to a boil, then lower the heat and simmer gently, partly covered, for 30 minutes, until the broth tastes of onion all the way through.
Fish out the bay leaves and thyme stalks. Taste and salt: homemade unsalted stock may need 1–2 tsp, a shop-bought one may need none. If you're using the cognac, take the pot off the heat, stir it in, then return it to a gentle simmer for 1 minute to soften the alcohol.

Toast and gratinée

While the soup simmers, heat the oven to 180°C (160°C fan). Lay the baguette slices on a baking tray and bake for 12–15 minutes, turning once, until dry and crisp all the way through and lightly golden. Rub one side of each slice with the cut garlic.
Switch to the grill (broiler) on high and set a rack so the tops of your bowls will sit about 10 cm below the heat. Stand 4 ovenproof soup bowls (about 400 ml each) on a sturdy baking tray.
Bring the soup back to a simmer and ladle it into the bowls, filling them to about 1 cm below the rim. Float 2 toasts on each, garlic side up, then pile on the cheese, letting it reach the edges of the bowls so it can stick and crisp there.
Slide the tray under the grill for 3–5 minutes, until the cheese has melted, bubbled and browned in patches. Stay close: it goes from golden to burnt in under a minute.
Lift the tray out with oven gloves. The bowls are now as hot as the grill and stay that way for a long time, so set each one on a plate or folded napkin before it goes to the table and warn everyone. Let the soup stand for 2 minutes before eating; the cheese traps a lot of steam.

Swaps

Cooking for coeliac disease or an allergy? Check every label: the notes below say which ones matter.

Gluten-free

Use 15 g rice flour in place of the plain flour, cooked exactly as in step 4, or leave it out for a slightly thinner broth.
Use a loaf labelled gluten-free instead of the baguette, sliced and dried in the oven as in step 8 until crisp right through.
Check the stock: stock cubes, bouillon and some cartons contain wheat or barley. The wine, cognac, Gruyère and Comté are gluten-free.

Plant-based

Use 1.5 l dark mushroom stock (check the label) and olive oil in place of the butter; the onions brown just as well, though the soup is a little less rich. Some wines are fined with egg, milk or fish products, so choose one labelled vegan, and check the baguette is a plain one of flour, water, yeast and salt.
The gratin is the hard part. Vegan cheeses melt and brown very differently, so test one on a toast under the grill first, or skip the cheese and float the garlic toasts on top, brushed with olive oil. It's still a very good onion soup, but no longer a gratinée.

High-protein

About 20 g protein a bowl as written, mostly from the cheese. For about 25 g more, stir 80–100 g shredded cooked beef per bowl into the soup before you fill the bowls, such as the shin or brisket left from making your stock.
Use 300 g cheese instead of 200 g for about 7 g more per bowl, and a thicker crust.

Tips

Slice evenly and use ordinary onions. Slices of the same thickness brown at the same rate, so you don't get burnt slivers among pale ones, and cutting root to tip helps them hold some shape. Yellow (brown) onions have enough sugar to brown deeply and enough bite to keep the soup savoury.
Deglaze early, not late. The brown film on the pot is pure flavour right up until it turns black, and then it's bitterness. A splash of water every time it darkens is how that colour ends up in the onions instead of in the washing-up.
Stock is the other half of the soup. A homemade beef stock, with some body from the bones, makes the soup taste rich rather than salty. If you buy it, choose a low-salt one so you control the seasoning. A good chicken stock makes a lighter but still excellent soup; dark mushroom stock is the vegetarian route (see Variations).
Dry the bread right through. Toast that is crisp to the centre holds up under the cheese; soft bread turns to paste the moment it meets the broth.
Use bowls made for the grill. Not every ceramic bowl can take direct heat. Use bowls sold as ovenproof or grill-safe, and never put a cold bowl straight under a hot grill.
Know your cheese. Gruyère (from Switzerland) and Comté (from the Jura in eastern France) are firm, nutty mountain cheeses that melt smoothly and brown well. Any similar Alpine-style cheese works; avoid pre-grated cheese coated in anti-caking powder, which melts grainy.

Make ahead and storage

The caramelised onions keep for 4 days in the fridge or 3 months in the freezer, so it's worth doing a double batch.
The soup itself (without bread and cheese) is even better the next day. Cool it quickly, then refrigerate for up to 3 days or freeze for up to 3 months. Reheat to a full simmer before filling the bowls.
Toast the baguette up to 2 days ahead and keep it airtight.

Variations

Vegetarian: use 1.5 l dark mushroom stock in place of the beef stock and choose a cheese made with non-animal rennet; traditional Gruyère and Comté are made with animal rennet.
Cognac instead of wine: deglaze with 150 ml water in step 5, then finish with 3 tbsp cognac in step 7.
Without ovenproof bowls (shortcut): heap the cheese on the toasts, grill them on a tray until bubbling, and float them on bowls of hot soup. You lose the crust on the rim, but not much else.

Reviews & cooks' tips

Reviews and tips from people who've cooked this appear here.