French Onion Soup

Ingredients

For the soup

• 4 tbsp unsalted butter
• 3½ lb yellow onions, about 7 medium, halved and thinly sliced from root to tip
• 2 tsp Diamond Crystal kosher salt
• water, for scraping up the pot
• ½ cup dry white wine
• 4 cups low-sodium beef stock
• 4 cups low-sodium chicken stock
• 4 sprigs thyme
• 1 bay leaf
• ½ tsp black pepper, freshly ground
• 2 tbsp dry sherry

For the top

• 1 baguette, cut into 12 slices about 3/4 inch thick
• 1 clove garlic, halved
• 8 oz Gruyère, coarsely grated, about 2 1/2 cups
• 1 oz Parmesan, finely grated, about 1/2 cup

The salt amounts here are for Diamond Crystal kosher salt. If you have Morton kosher salt, use a little over half as much, because its grains are denser and saltier by the spoon.

Steps

1. Melt the butter in a heavy 6- to 8-quart Dutch oven over medium heat. Add the onions and salt and toss until coated; the pot will be heaped. Cover, lower the heat to medium-low and leave them alone for 15 minutes, until they have collapsed by about half and released a lot of liquid.
2. Uncover and turn the heat back to medium. Cook, stirring every 5 minutes or so, until the onions are a deep chestnut brown, soft enough to smear, and down to roughly a quarter of their starting volume, 60 to 75 minutes. Once the liquid has cooked off, a brown film will start to form on the bottom of the pot: when it darkens, add a splash of water and scrape it up into the onions. Expect to do this 6 or 8 times. If the film goes black rather than brown, lower the heat.
3. Pour in the wine and scrape the bottom clean. Cook until the wine has almost disappeared, 2 to 3 minutes.
4. Add the beef stock, chicken stock, thyme, bay leaf and pepper. Bring to a boil, then lower to a gentle simmer, partly covered, for 20 minutes. Pull out the herb stems and bay leaf, turn off the heat, and stir in the sherry. Taste for salt.
5. While the soup simmers, heat the oven to 400°F. Lay the baguette slices on a sheet pan and bake until dry and crisp all the way through, 10 to 12 minutes, turning once. Rub one side of each slice with the cut side of the garlic.
6. Move a rack to about 6 inches below the broiler and heat the broiler to high. Set six ovenproof bowls on a sheet pan and fill them with the hot soup. Float two slices of toast on each, then pile the Gruyère on top, taking it right to the rims, and finish with the Parmesan.
7. Broil until the cheese is bubbling and browned in spots, 3 to 5 minutes. Let the bowls sit 5 minutes before serving; they come out dangerously hot.

Notes

The soup, without bread or cheese, keeps 4 days in the fridge or 3 months in the freezer, and the onions alone can be browned up to 3 days ahead. If your bowls can't go under the broiler, broil the cheese on the toasts on a sheet pan and float them on the soup. Comté is just as good as the Gruyère here.