Step 1: Caramelize the Onions
Melt the butter in a heavy Dutch oven over medium heat. Add the onions and ½ teaspoon of salt, give them a good stir, and cover the pot. Let the onions soften for 5 minutes.

Remove the lid, lower the heat slightly, and let the onions caramelize, stirring occasionally, until they turn a deep golden brown. This process is slow and steady—45 to 60 minutes—but it’s absolutely worth it. Adjust the heat if they’re browning too quickly.

Step 2: Warm the Broth
While the onions are caramelizing, warm the chicken and beef broths in a separate saucepan over low heat.

Step 3: Deglaze and Build the Soup
Once the onions are perfectly caramelized, add the white wine and sherry to the pot, scraping up any golden bits on the bottom. Let it simmer for a minute or two until the liquid reduces slightly.

Sprinkle the flour over the onions, stir well to combine, and let it cook for 1-2 minutes to thicken.

Step 4: Simmer the Broth
Slowly pour in the warm chicken and beef broths, stirring constantly. Add the remaining ¼ teaspoon salt and the black pepper. Bring the soup to a gentle boil, then reduce the heat and let it simmer uncovered for 10 minutes. Taste and adjust seasoning—you want it perfectly balanced.

Step 5: Assemble and Broil
Preheat the broiler. Arrange ovenproof bowls or casseroles on a baking sheet.

Ladle the soup into the bowls, leaving about an inch of space at the top. Place 1 to 2 slices of toasted French bread on top of the soup, then generously sprinkle each bowl with grated Gruyère.

Step 6: Melt the Cheese
Slide the bowls under the broiler and watch carefully—this only takes 1 to 2 minutes. You want the cheese to melt, bubble, and turn golden brown.

Step 7: Serve Immediately
Carefully remove the bowls from the oven (they’ll be piping hot!) and serve immediately. A little sprinkle of flaky sea salt or a fresh crack of black pepper over the top wouldn’t hurt.