Heat a large, heavy-bottomed pot over medium heat. Add 3 tablespoons unsalted butter and let it melt. Add 10 cups thinly sliced onions and ¾ teaspoon salt and stir.

Let the onions sauté for 5-10 minutes or until they begin to turn translucent. Reduce the heat to medium-low and continue to cook the onions for another 45 minutes, stiring every 5-10 minutes to prevent them from burning. If they are burning instead of caramelizing, turn the heat to low and stir more frequently. They should be fragrant and turn a deep golden brown. The size will reduce drastically.

Turn the heat back to medium, stir 2 tablespoons all-purpose flour into the onions, and cook for 1 minute. Slowly stir in 4 cups beef broth , ¼ cup sherry, and ½ cup dry white wine. Bring to a simmer. Simmer until the soup begins to thicken and reduce, about 4-5 minutes. Stir in 5 sprigs thyme. Adjust the salt as needed.

While the onions cook, toast the 4-8 slices baguette pieces in the oven. Brush both sides with olive oil, then bake at 350 degrees Fahrenheit for 6-8 minutes. Flip the bread halfway through. Remove from the oven and set aside.

Ladle the soup into four oven-safe bowls. Place a slice or two of toasted bread on top of the soup and cover with some of the 8 ounces Gruyere cheese. Place the bowls on a large sheet pan and broil until the cheese has melted and started to turn brown and bubbly, about 2-3 minutes. Keep a close eye on them. Some broilers can cook faster than others. Serve immediately.