Spinach Cheese Strata

Name: Spinach & Cheese Strata
Ingredients: 1 10 oz. package frozen spinach, thawed; 1 large yellow onion, diced; 2 tablespoons butter; 1 teaspoon salt; 2 dashes ground black pepper; 1 dash ground nutmeg; 1 loaf ciabatta, cubed; 1 cup gruyere, grated; 1 cup sharp white cheddar, grated; 2 1/2 cups low-fat milk; 8 large eggs