01 - Preheat oven to 400°F. Line a baking tray with parchment paper and set aside.
02 - In a large mixing bowl, combine ground beef, egg, breadcrumbs, Parmesan cheese, minced garlic, grated onion, chopped parsley, salt, and black pepper. Mix until evenly incorporated without overworking 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 tightly around the cheese. Seal edges well and roll into a smooth ball.
04 - Arrange the stuffed 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 whole milk, stirring continuously until the mixture thickens and coats the back of a spoon, about 2–3 minutes.
07 - Reduce heat to low. Stir in cheddar cheese, pepper jack cheese, smoked paprika, cayenne pepper, hot sauce, and salt. Continue stirring until all cheese is melted and the sauce is smooth and creamy.
08 - Transfer the hot meatballs to a serving platter and drizzle generously with spicy cheese sauce. Garnish with fresh chopped parsley. Serve alongside crusty bread or over pasta.