01 - Preheat oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - In a large mixing bowl, combine the ground beef, egg, breadcrumbs, Parmesan cheese, minced garlic, grated onion, chopped parsley, salt, and pepper. Mix gently with your hands until all ingredients are evenly distributed without overworking the meat.
03 - Divide the meat 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. Press the seams firmly to seal, rolling into a smooth ball.
04 - Arrange the stuffed meatballs on the prepared baking sheet with even spacing. Bake for 20 to 25 minutes until deeply browned on the outside and cooked through to an internal temperature of 160°F.
05 - While the meatballs bake, melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly to form a smooth blond roux.
06 - Gradually pour in the whole milk while whisking vigorously to prevent lumps. Continue cooking and whisking for 2 to 3 minutes until the mixture thickens enough to coat the back of a spoon.
07 - Reduce heat to low. Add the shredded cheddar, pepper jack, smoked paprika, cayenne pepper, hot sauce, and salt. Stir continuously until all the cheese has melted and the sauce is silky smooth.
08 - Transfer the hot meatballs to a serving platter and drizzle generously with the spicy cheese sauce. Garnish with freshly chopped parsley. Serve alongside crusty bread or over cooked pasta.