1. Preheat your oven to 375°F (190°C). Grease a baking dish lightly with cooking spray.
2. In a skillet, cook the ground beef and onion over medium heat until the beef is browned and the onion is soft. Drain excess grease. Season with garlic powder, paprika, salt, and pepper.
3. In a small bowl, mix mayonnaise, ketchup, and mustard to create the sauce.
4. Layer the beef mixture in the prepared baking dish. Spread the sauce evenly on top, then sprinkle with crumbled bacon.
5. Top with cheddar and mozzarella cheese.
6. Bake for 15-20 minutes, or until the cheese is bubbly and golden.
7. Let cool slightly before serving, garnished with fresh parsley if desired.