Slow Cooker French Onion Soup

Name: Slow Cooker French Onion Soup
Prep time: 30 min
Cook time: 4 h 45 min
Total time: 5 h 15 min
Cuisine: French
Ingredients: 6 tablespoons butter; 4 large yellow onions, sliced and separated into rings; 1 tablespoon white sugar; 2 cloves garlic, minced; 0.5 cup cooking sherry; 7 cups reduced-sodium beef broth; 1 teaspoon sea salt, or to taste; 0.25 teaspoon dried thyme; 1 bay leaf; 8 slices of French bread; 0.5 cup shredded Gruyere cheese; 0.33333334326744 cup shredded Emmental cheese; 0.25 cup freshly shredded Parmesan cheese; 2 tablespoons shredded mozzarella cheese