Preheat oven to 350°F (175°C).Lightly grease a 9×13-inch baking dish with olive oil.
Whisk the base.In a large mixing bowl, whisk together the eggs, milk, salt, and black pepper until well combined.
Add vegetables and seasonings.Stir in the chopped bell peppers, spinach, diced tomatoes, onion, garlic powder, and oregano until evenly mixed.
Pour into baking dish.Transfer the mixture into the greased baking dish and spread evenly.
Top with cheese.Sprinkle the shredded cheese evenly over the top of the casserole.
Bake.Bake uncovered for 30–35 minutes, or until the eggs are set and the top is lightly golden.
Cool and serve.Let the casserole rest for 5–10 minutes before slicing. Serve warm and enjoy!