# French Onion Soup French onion soup is a rich, sweet and savory broth full of slowly caramelized onions, topped with a toasted baguette and melted Gruyere. It looks fancy but only needs patience, not skill. The flavor comes from the onions, so give them the full time. Done right, a pile of sliced onions shrinks to a small, dark, jammy heap that makes the whole pot taste deeply sweet. - Total time: 1 h 35 min (prep 20 min, cook 1 h 15 min) - Serves: 4 - Per serving (estimated): 480 kcal, 22 g protein - Cuisine: French. Category: Soup. ## Ingredients - 4 tbsp butter - 1 tbsp olive oil - 1 kg yellow onions, thinly sliced - 1 tsp salt, divided - 3 cloves garlic, minced - 1 tbsp all-purpose flour - 1/2 cup dry white wine - 6 cups beef broth - 4 sprigs fresh thyme - 2 bay leaves - 1 tbsp sherry vinegar - 1/2 tsp black pepper - 8 slices baguette, 1.5 cm thick - 200 g Gruyere cheese, grated ## Method 1. Melt the butter with the oil in a large Dutch oven over medium heat. Add the onions and 1/2 tsp salt. 2. Cook 45 to 55 minutes, stirring every 5 minutes, until the onions are deep golden brown, soft and jammy. Lower the heat if they start to burn. 3. Stir in the garlic and flour and cook 1 minute, until the flour smells slightly toasty and no white streaks remain. 4. Pour in the wine and scrape the browned layer off the bottom. Simmer 2 minutes until almost dry. 5. Add the broth, thyme, bay leaves and remaining 1/2 tsp salt. Bring to a boil, then simmer gently 20 minutes. 6. Remove the thyme stems and bay leaves. Stir in the vinegar and pepper, then taste and adjust. 7. Heat the broiler to high. Toast the baguette slices on a tray for 2 minutes per side, until dry and lightly golden. 8. Ladle the soup into 4 oven-safe bowls on a baking sheet. Top each with 2 slices of bread and a thick layer of Gruyere. Broil 3 to 4 minutes, until bubbling with brown spots. ## Why it works - Slow cooking breaks down the onions and browns their natural sugars, which creates the deep, sweet flavor. - A little flour cooked in the onions thickens the broth just enough to feel silky. - Deglazing with wine lifts the browned layer off the pot, and that layer is concentrated flavor. - A splash of vinegar at the end balances the sweetness so the soup tastes bright and not cloying. ## The science - **Caramelizing is slow chemistry.** Onions are about 90 percent water. As it cooks off, their sugars concentrate and brown through caramelization and Maillard reactions. This takes time. Higher heat only scorches the edges, which is why a steady medium heat and frequent stirring work best. - **Why onions turn sweet.** Raw onions taste sharp because of sulfur compounds. Heat breaks those down and releases sugars, so the flavor turns mellow and sweet. Slicing thin and cooking until jammy means more surface area, which makes the transformation faster and more even. ## Tips - Slice the onions pole to pole so they hold their shape and cook evenly. - Add a splash of water whenever the bottom of the pot looks dry and the onions start to stick. - Use real stock or a good broth, since there are few ingredients to hide behind. ## Mistakes to avoid - Rushing the onions on high heat: they burn instead of caramelizing, so keep the heat at medium and stir often. - Using soggy bread: it dissolves, so toast the baguette until fully dry first. - Skipping the vinegar: the soup tastes too sweet, so finish with a little acid. ## Variations and swaps - Use a mix of Gruyere and Parmesan for a saltier, nuttier crust. - Add a tablespoon of brandy or cognac with the wine for deeper flavor. - Make it vegetarian with mushroom or vegetable broth and a spoonful of miso. - Swap Gruyere for Swiss or provolone, which melt well but taste milder. - Swap the white wine for 1/2 cup extra broth plus 1 tsp extra vinegar for an alcohol-free version. ## Storage Store the soup without the bread and cheese for up to 4 days in the fridge or 3 months in the freezer. Reheat to 165°F (74°C), then add the toast and cheese and broil. ## FAQ **How long does it take to caramelize onions for French onion soup?** It takes about 45 to 55 minutes over medium heat. The onions should be deep golden brown, soft and jammy before you add any liquid. **What is the best cheese for French onion soup?** Gruyere is the classic choice because it melts smoothly and has a nutty flavor. Swiss, provolone or a mix with Parmesan also work well. **What kind of onions are best?** Yellow onions are best because they have plenty of sugar and a balanced flavor. Sweet onions work but give a sweeter soup, while red onions can taste muddy. **Can I make French onion soup ahead?** Yes. Make the soup up to 3 days ahead and reheat it, then add the toast and cheese just before serving so the bread stays crisp. --- Source: https://cookingscientist.com/recipes/french-onion-soup (Cooking Scientist). Learn the science of cooking in the Cooking Scientist app for iPhone and iPad: https://apps.apple.com/app/id6830326846