Preheat the oven to 350°F (175°C). Lightly grease a 9×13 inch casserole dish with olive oil.
In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the diced onion and red bell pepper and cook until softened, about 5 minutes.
Add the ground beef to the skillet with the vegetables. Cook, breaking up the meat with a spoon, until browned and cooked through, about 7-8 minutes. Drain any excess fat.
Stir the chopped spinach into the beef mixture and cook for an additional 2 minutes until wilted. Remove from heat and set aside.
In a large bowl, whisk together the eggs, whole milk, salt, black pepper, garlic powder, and dried oregano until well combined.
Spread the beef and vegetable mixture evenly in the prepared casserole dish.
Pour the egg mixture evenly over the beef and vegetables.
Sprinkle the shredded cheddar cheese evenly over the 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.