Preheat your oven to 375˚F. Consider lining a large baking sheet with parchment paper if it's not nonstick.
Heat 2 tablespoons of oil or butter in a skillet over medium heat. Once hot, add hash browns and cook for about 10 minutes until crispy.
In a separate skillet, heat the remaining 1 tablespoon of oil, add onion, and cook for 1 minute. Then, add bell pepper, cook for 2-3 minutes until softened, and toss in spinach until wilted. Remove from heat.
Whisk eggs, milk, salt, pepper, garlic powder, onion powder, oregano, and parsley in a large bowl.
Spread the cooked vegetables (including hash browns) and ⅓ of the feta on the sheet pan. Pour the egg mixture over and stir gently. Sprinkle the remaining feta on top and add red pepper flakes if desired.
Bake for 20 minutes or until the eggs set. Let it cool a bit before slicing and serving.