Make-Ahead Spinach and Mozzarella Breakfast Strata Recipe

Ingredients

• 3 tablespoons unsalted butter
• 1.5 cups finely chopped yellow onion
• 3 pinches herbes de Provence, or to taste
• salt and ground black pepper to taste
• 2 (10 ounce) packages frozen chopped spinach, thawed and drained
• butter-flavored cooking spray
• 8 cups cubed Italian bread
• 1 cup shredded reduced-fat mozzarella cheese, or more to taste
• 9 large eggs
• 1 cup skim milk