Melt butter in a Dutch oven over medium heat. Add onions and ½ teaspoon salt, stir, and cover, letting onions soften for 5 minutes. Remove the lid and let onions caramelize until golden brown over medium heat, stirring occasionally. Adjust heat if onions are browning too quickly. The caramelization process may take 45 to 60 minutes.
3 Tbsp Butter, 2 ea Yellow onions, thinly sliced, 2 ea Red onions, thinly sliced, 3/4 tsp Sea salt
Warm the broth in a saucepan over low heat.
64 oz Beef stock
Once onions are caramelized, add wine and sherry to the pot and allow the mixture to come to a boil. Stir in flour and let thicken for a minute or two.
1 cup Dry white wine, 1 Tbsp Dry sherry, 1 Tbsp Flour
Slowly add warm broth, ¼ teaspoon salt, and the pepper to the onion mixture and boil uncovered for 10 minutes. Add more salt and pepper to taste.
1/2 tsp Black pepper
Heat the broiler, and arrange individual ovenproof casseroles on a baking sheet. Ladle soup into casseroles, and cover the top with bread slices. Sprinkle each casserole generously with Gruyère. Broil for a minute or two, watching carefully, until the cheese melts and browns. Serve immediately.
1 ea French bread, sliced to 1/2", 1-1/2 cup Gruyère cheese, grated