French onion soup turns a large pile of ordinary onions into a deep, savoury broth. The flavour comes from patient browning, followed by a gentle simmer with stock and thyme. Toasted bread and melted Gruyère add a crisp, rich finish. This wine-free version uses a little cider vinegar to brighten the broth.
Photo by sheri silver / Unsplash. Serving suggestion. Presentation may vary.
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French Onion Soup with Gruyère Toasts
Ingredients
- 1 kg yellow onions, peeled and thinly sliced
- 40 g unsalted butter
- 1 tablespoon olive oil
- 1/2 teaspoon fine salt, plus more to taste
- 2 garlic cloves, finely chopped
- 1 tablespoon plain / all-purpose flour, about 10 g
- 1.2 litres low-salt beef stock
- 1 bay leaf
- 3 small thyme sprigs
- 2 teaspoons cider vinegar
- 1/4 teaspoon ground black pepper
- 8 baguette slices, about 120 g total
- 120 g Gruyère cheese, grated
Preparation
- Melt butter with oil in a large, heavy saucepan over medium-low heat. Add onions and salt. Cook uncovered for 40–45 minutes, stirring every few minutes, until soft and deeply golden. Add a tablespoon of water whenever browned deposits begin sticking hard to the base.
- Stir in garlic for 1 minute, then flour for another minute. Pour in a little stock and scrape up the browned flavour from the base. Gradually add the remaining stock, stirring.
- Add bay and thyme. Simmer gently for 20 minutes. Remove the herbs, stir in vinegar and pepper, and taste before adding more salt.
- While the soup simmers, heat the oven to 200°C conventional / 180°C fan. Toast baguette slices on a tray for 6–8 minutes until crisp, turning once.
- Top the toasts with Gruyère. Switch to a hot grill / broiler and melt the cheese for 2–4 minutes, watching continuously. Divide hot soup among four bowls and add two cheese toasts to each just before serving.
Slicing onions for even cooking
Cut onions into reasonably even half-moons rather than very fine fragments. Thick and thin pieces mixed together brown at different speeds, making it harder to develop a consistent base. A wide heavy saucepan provides space for moisture to evaporate. The onions may initially fill most of the pan, but they shrink considerably. Keep the lid off during browning so the water can escape instead of collecting and steaming the onions.
Caramelising without burning
The colour should build gradually from pale gold to a deeper amber. Stir more frequently as the onions lose moisture. Brown deposits on the base are useful flavour, while black specks and an acrid smell mean the heat is too high. Add small splashes of water to dissolve sticky patches and keep the process moving. Some batches take longer because onions vary in water content. Extend the browning time rather than raising the heat sharply to meet the clock.
Choosing stock and balancing salt
A low-salt stock allows room for the cheese and bread, both of which contribute seasoning. Beef stock gives a full savoury flavour. A rich vegetable stock can be used if preferred, but choose vegetarian cheese as well when serving a vegetarian meal. Avoid assuming that all Gruyère is vegetarian, since the rennet can differ. Taste the broth after the simmer because evaporation concentrates both salt and flavour.
Preparing the cheese toast
Drying the bread in the oven helps it keep some texture on the soup. Melt the cheese separately on a tray, which avoids putting ordinary serving bowls beneath a grill. Use only cookware specifically rated for a grill if choosing a traditional bowl-topped finish. Keep an eye on the tray because melted cheese can brown quickly. Add the toasts at the table if the soup needs to wait, otherwise they will soak through.
Serving and allergens
This is a warming lunch or starter, with four moderate portions. Serve it with a simple salad for a fuller meal. The recipe contains milk and wheat. Stock may contain celery or other allergens, so check the packaging. Cider vinegar supplies acidity and does not require adding wine. The soup should taste savoury with a gentle onion sweetness rather than strongly sour.
Make-ahead storage
Cool the soup promptly in shallow containers and refrigerate within 2 hours. Use within 3 days or freeze without the bread and cheese for up to 2 months. Thaw overnight in the fridge and reheat until steaming hot throughout. Prepare the toasts fresh while the soup reheats. Keep any leftover cheese toasts separately and refresh briefly in the oven.
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