01 - Preheat oven to 400°F. Line a baking tray with parchment paper and set aside.
02 - In a large bowl, combine ground beef, egg, breadcrumbs, Parmesan, garlic, grated onion, parsley, salt, and pepper. Mix until evenly incorporated, being careful not to overwork the meat.
03 - Divide the mixture into 12 equal portions. Flatten each portion in the palm of your hand, place a mozzarella cube in the center, and wrap the meat around the cheese. Seal edges firmly to form a tight ball, ensuring no gaps remain.
04 - Arrange the meatballs on the prepared baking tray. Bake for 20–25 minutes until golden brown and cooked through to an internal temperature of 160°F.
05 - While the meatballs bake, melt butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly to form a smooth roux.
06 - Gradually whisk in the milk, pouring slowly while stirring to prevent lumps. Continue cooking for 2–3 minutes until the mixture thickens enough to coat the back of a spoon.
07 - Reduce heat to low. Add cheddar, pepper jack, smoked paprika, cayenne, hot sauce, and salt. Stir continuously until all cheese is melted and the sauce is smooth and creamy.
08 - Transfer hot meatballs to a serving platter and drizzle generously with spicy cheese sauce. Garnish with fresh parsley. Serve alongside crusty bread or over pasta.