Preheat oven to 350°F (175°C).
In a large skillet, heat olive oil over medium heat. Add diced bell pepper and diced onion; sauté for 5 minutes until softened.
Add chopped spinach to the skillet and cook for an additional 2 minutes until wilted. Remove from heat and set aside.
In a large bowl, whisk together eggs, whole milk, salt, black pepper, garlic powder, and onion powder until well combined.
Add cooked sausage, sautéed vegetables, shredded cheddar cheese, and shredded mozzarella cheese to the egg mixture. Stir gently to combine.
Lightly grease a 9×13-inch baking dish. Pour the egg mixture evenly into the dish.
Cover the casserole tightly with aluminum foil.
To freeze: Place the covered casserole in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 350°F (175°C). Remove foil and cover casserole with fresh foil. Bake for 45 minutes covered, then remove foil and bake an additional 15 minutes until eggs are set and top is golden.
To bake from thawed: Thaw casserole overnight in the refrigerator. Preheat oven to 350°F (175°C). Bake uncovered for 40-45 minutes until eggs are set and top is golden.
Let casserole rest for 5 minutes before slicing and serving.