- 3 tbsp. unsalted butter
- 3 to 4 large Vidalia onions (about 3 lbs), peeled and thinly sliced
- 1 tsp. sugar
- 1 tsp. garlic
- 1/2 tsp. dried thyme
- 3/4 tsp. kosher salt (divided)
- 4 c beef stock
- 1/2 c chicken broth
- 3/4 c dry white wine
- 1 tbsp. dry sherry
- 1 tbsp. all-purpose flour
- 1/2 tsp. black pepper, more to taste
- 2 bay leaves
- 1 tbsp. Worcestershire sauce
- 8 to 12 French bread slices cut 1/2" thick
- 1 1/2 c grated Gruyère cheese
- Melt butter in a heavy Dutch oven over medium heat.
- Add onions and ½ teaspoon salt, stir and cover, letting onions soften for 5 minutes.
- Remove lid and let onions caramelize until golden brown over medium heat, stirring occasionally.
- Add sugar and continue cooking. Adjust heat if onions are browning too quickly. The caramelization process may take 45 to 60 minutes.
- Add garlic and thyme and cook for 25 minutes.
- Warm beef stock and chicken broth in a saucepan over low heat.
- Once onions are caramelized, add wine and sherry to the pot and allow mixture to come to boil. Stir in flour and let thicken for a minute or two.
- Slowly add warm broth mixture, ¼ teaspoon salt, pepper and bay leaves to the onion mixture and boil uncovered for 10 minutes.
- Add Worcestershire sauce and add more salt and pepper, to taste.
- Heat the broiler and arrange individual ovenproof casseroles on a baking sheet. Ladle soup into casseroles, and cover top with bread slices. Sprinkle each casserole generously with Gruyère. Broil for a minute or two, watching carefully, until cheese melts and browns.
- Serve immediately.