Crock pot french onion soup

Ingredients

• 2 tablespoons unsalted butter
• 6 medium sweet onions, thinly sliced
• 1 clove garlic, minced
• 3 cans beef broth (14 oz each)
• 2 teaspoons Worcestershire sauce
• 1/4 teaspoon freshly cracked black pepper, or to taste
• 1 cup water
• 1/4 cup dry sherry, optional
• 8 slices French bread, 1 inch thick
• 8 slices Swiss or Gruyere cheese