Brown sausage in a large skillet over medium, breaking into crumbles; add onion and pepper and cook about 5 minutes until softened. Stir in spinach until wilted; drain excess grease and cool.
Spray a 9x13-inch dish; spread bread evenly.
Add sausage mixture, mozzarella, and Parmesan.
Whisk eggs, milk, Italian seasoning, salt, and pepper in a large bowl.
Pour evenly over filling; press gently to soak bread.
Cover tightly with plastic and foil; freeze up to 3 months or refrigerate overnight.
If frozen, thaw refrigerated 24 hours; remove from fridge 30 minutes before baking. Heat oven to 350°F.
Remove plastic, cover with foil, and bake 30 minutes, then uncovered 15 minutes until golden and eggs set in center.