Slow Cooker French Onion Chicken

Ingredients

• 4 boneless, skinless chicken breasts
• 2 large yellow onions, thinly sliced
• 3 tbsp unsalted butter
• 1 tbsp olive oil
• 2 cloves garlic, minced
• 1 cup beef broth
• 1 tbsp Worcestershire sauce
• 1 tsp balsamic vinegar
• 1 tsp salt
• 1/2 tsp black pepper
• 1/2 tsp dried thyme
• 1/2 tsp onion powder
• 1/2 tsp garlic powder
• 1/2 cup heavy cream (optional for a creamier sauce)
• 1 cup shredded Gruyère cheese (or Swiss cheese)
• 1/2 cup shredded mozzarella cheese
• 1/4 cup grated Parmesan cheese
• 4 slices French bread, toasted (for serving, optional)
• 1 tbsp fresh parsley, chopped (for garnish)