01 - Preheat the oven to 400°F. Line a baking tray with parchment paper.
02 - In a large bowl, combine the ground beef, egg, breadcrumbs, Parmesan, garlic, grated onion, parsley, salt, and pepper. Mix until evenly incorporated.
03 - Divide the mixture into 12 equal portions. Flatten each portion in your palm, place a mozzarella cube in the center, and wrap the meat tightly around the cheese, sealing well to form a ball.
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 the butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute to form a roux.
06 - Gradually whisk in the milk, stirring constantly until the mixture thickens, about 2–3 minutes.
07 - Reduce heat to low. Stir in the cheddar, pepper jack, smoked paprika, cayenne, hot sauce, and salt. Continue stirring until the cheeses are fully melted and the sauce is smooth.
08 - Arrange the hot meatballs on a serving platter and drizzle generously with spicy cheese sauce. Garnish with fresh parsley and serve alongside crusty bread or pasta.