Bubbling gruyere cheese and crispy baguette top a brothy base full of perfectly braised onions in this showstopping French onion soup recipe. Whether you're making it to warm up on a cold day or for a delicious date night in, this feels luxurious and cozy each and every time.
Braise/Caramelize Onions: Adjust oven rack to lower-middle position and heat oven to 400 degrees F. Generously spray inside of heavy-bottomed large (at least 7-quart) Dutch oven with nonstick cooking spray. Place butter in the pot and add onions and 1 teaspoon salt. Cook, covered, 1 hour (onions will be moist and slightly reduced in volume). Remove pot from oven and stir onions, scraping bottom and sides of pot. Return pot to oven with lid slightly ajar and continue to cook until onions are very soft and golden brown, 1½ to 1¾ hours longer, stirring onions and scraping bottom and sides of pot after 1 hour.
Carefully remove pot from oven and place over medium-high heat. Using oven mitts to handle pot, cook onions, stirring frequently and scraping bottom and sides of pot, until liquid evaporates and onions brown, 15 to 20 minutes, reducing heat to medium if onions are browning too quickly. Continue to cook, stirring frequently, until pot bottom is coated with dark crust, 6 to 8 minutes, adjusting heat if necessary. (Scrape any crust that collected on spoon back into onions.) Stir in ¼ cup water, scraping pot bottom to loosen crust, and cook until water evaporates and pot bottom has formed another dark crust, 6 to 8 minutes. Repeat process of deglazing 2 or 3 more times, until onions are very dark brown. Stir in sherry and cook, stirring frequently, until sherry evaporates, about 5 minutes.
Finish the Soup: Stir in broths, 2 cups water, thyme, bay leaf, and ½ teaspoon salt, scraping up any final bits of browned crust on bottom and sides of pot. Increase heat to high and bring to simmer. Reduce heat to low, cover, and simmer 30 minutes. Remove and discard herbs, then season with salt and pepper.
Broil with Cheesy Baguette Slices: While soup simmers, arrange baguette slices in single layer on baking sheet and bake in 400-degree oven until bread is dry, crisp, and golden at edges, about 10 minutes. Set aside.
Adjust oven rack 6 inches from broiler element and heat broiler. Set individual broiler-safe crocks on baking sheet and fill each with about 1¾ cups soup. Top each bowl with 1 or 2 baguette slices (do not overlap slices) and sprinkle evenly with Gruyere. Broil until cheese is melted and bubbly around edges, 3 to 5 minutes. Let cool 5 minutes before serving.
Onions: Use yellow onions for the best balance of flavor.
Broth: This recipe calls for a combination of beef and chicken broth; you can adjust the ratios based on what you have on hand and substitute stock if you'd like.
Baguette: Sliced French baguette is the traditional topping for French onion soup, but any crusty bread will suffice, and if you're in a pinch, you can use big croutons, too!
Cheese: Gruyere is the standard, but shredded Swiss makes a great substitute.
Broiling: If using ordinary soup bowls, sprinkle the toasted bread slices with Gruyère and return them to the broiler until the cheese melts, then float them on top of the soup.
Make-Ahead: For the best flavor, make the soup a day or 2 in advance. Alternatively, the onions can be prepared through step 1, cooled in the pot, and refrigerated for up to 3 days before proceeding with the recipe.
Storage: Leftover soup can be refrigerated for up to 4 days.
Freezing: The soup can be frozen in an airtight container for up to 3 months. Do not freeze the cheese-covered bread with the soup; freeze the soup separately and add the bread and cheese before serving.