Prepare the onions. Peel and thinly slice the Vidalia onions, then set them aside.
4 large sweet vidalia onions
Caramelize the onions. Melt the butter in a heavy-bottom pot over medium heat, then add the onions and coarse Kosher salt. Stir occasionally for up to an hour until they're beautifully browned and caramelized. This step takes patience, but the delicious end result is absolutely worth it.
3 Tablespoons unsalted butter, 1 Tablespoon coarse Kosher salt
Build the base. Sprinkle the flour over the caramelized onions and stir for 1-2 minutes until it forms a paste. Add in the white wine and scrape up all those browned bits from the bottom of the pot. Pour in the sherry and beef stock, then add the thyme and black pepper. Let this simmer to bring all the flavors together.
2 Tablespoons all-purpose flour, 8 cups beef stock, 1 cup white wine, 1 Tablespoon dry sherry, 1 teaspoon fresh thyme leaves, cracked black pepper
Top and broil. Ladle soup into oven-safe bowls. Place a slice of French bread on top of each bowl, then generously top with gruyere cheese. Place bowls on a baking sheet and slide under the broiler until the cheese is bubbly and golden brown. Keep a close eye on it because this happens quickly.
1 loaf French bread, 2 cups Gruyère cheese
Serve and enjoy. Serve immediately while the cheese is still bubbling, and enjoy the combination of sweet caramelized onions, rich broth, and melted gruyere.