Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat.
Add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Add ground beef to the skillet and cook until browned, breaking it apart with a spatula, about 6-8 minutes.
Drain excess fat from the skillet.
Stir in frozen mixed vegetables, salt, black pepper, paprika, dried oregano, and crushed red pepper flakes.
Cook for 5 minutes until vegetables are heated through and flavors are combined.
In a large mixing bowl, combine thawed hashbrowns, sour cream, and milk. Mix well.
Lightly grease a 9×13 inch casserole dish.
Spread half of the hashbrown mixture evenly on the bottom of the casserole dish.
Layer the cooked ground beef and vegetable mixture evenly over the hashbrowns.
Sprinkle half of the shredded cheddar cheese over the beef layer.
Spread the remaining hashbrown mixture evenly over the cheese layer.
Top the casserole with shredded mozzarella cheese and the remaining cheddar cheese.
Cover the casserole dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes until the cheese is melted and bubbly and the top is golden brown.
Remove from oven and let the casserole rest for 5 minutes before serving.