6 tablespoons butter

4 large yellow onions, sliced and separated into rings

1 tablespoon white sugar

2 cloves garlic, minced

1/2 cup cooking sherry

7 cups reduced-sodium beef broth

1 teaspoon sea salt, or to taste

1/4 teaspoon dried thyme

1 bay leaf

8 slices of French bread

1/2 cup shredded Gruyere cheese

1/3 cup shredded Emmental cheese

1/4 cup freshly shredded Parmesan cheese

2 tablespoons shredded mozzarella cheese