Slow Cooker French Onion Soup

INGREDIENTS

• 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