Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or olive oil.
In a large mixing bowl, whisk together the eggs, milk, salt, pepper, garlic powder, and onion powder until well combined.
In a skillet, heat the olive oil over medium heat. Add the diced bell pepper and onion, and sauté for about 5 minutes, or until softened.
Add the cooked turkey and spinach to the skillet, stirring to combine. Cook for an additional 2-3 minutes until heated through.
Pour the turkey and vegetable mixture into the prepared baking dish, spreading it evenly.
Pour the egg mixture over the turkey and vegetables, ensuring everything is evenly coated.
Sprinkle the shredded cheddar cheese on top.
Bake in the preheated oven for 30-35 minutes, or until the eggs are set and the top is lightly golden.
Let the casserole cool for about 5 minutes before slicing and serving.