Preheat the oven to 350°F (175°C). Lightly grease a 9×13 inch baking dish.
In a large skillet over medium heat, cook the ground beef and diced onion until the beef is browned and the onion is softened, about 7-8 minutes. Drain excess fat.
Add the diced red bell pepper and chopped spinach to the skillet. Cook for another 3-4 minutes until the vegetables are softened. Remove from heat and set aside.
In a large mixing bowl, whisk together the eggs, whole milk, salt, black pepper, garlic powder, and smoked paprika until well combined.
Spread the cooked ground beef and vegetable mixture evenly in the prepared baking dish.
Pour the egg mixture evenly over the beef and vegetables.
Sprinkle the shredded cheddar cheese evenly on top.
Bake in the preheated oven for 40-45 minutes, or until the eggs are set and the top is lightly golden.
Remove from the oven and let cool for 5 minutes before slicing and serving.