Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish with cooking spray or butter.
In a large skillet over medium heat, cook the ground beef until browned, breaking it apart with a spatula as it cooks, about 6-8 minutes.
Add the diced onion and red bell pepper to the skillet with the beef. Cook until the vegetables are softened, about 5 minutes. Drain excess fat if necessary.
Stir in the chopped spinach and cook for an additional 2 minutes until wilted. Remove the skillet from heat and set aside.
In a large mixing bowl, whisk together the eggs, whole milk, salt, black pepper, garlic powder, and dried oregano until well combined.
Spread the cooked beef and vegetable mixture evenly in the prepared baking dish.
Pour the egg mixture evenly over the beef and vegetables in the baking dish.
Sprinkle the shredded cheddar cheese evenly on top of the casserole.
Bake in the preheated oven for 40-45 minutes, or until the eggs are set and the top is golden brown.
Remove from the oven and let the casserole cool for 5 minutes before slicing and serving.