Slow Cooker French Onion Soup

Ingredients

• tablespoons butter
• large yellow onions, sliced and separated into rings
• tablespoon white sugar
• garlic, minced
• cooking sherry
• reduced-sodium beef broth
• teaspoon sea salt, or to taste
• teaspoon dried thyme
• bay leaf
• slices of French bread
• shredded Gruyere cheese
• shredded Emmental cheese
• freshly shredded Parmesan cheese
• tablespoons shredded mozzarella cheese