01 - Preheat oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - In a large mixing bowl, soak the breadcrumbs in 1/4 cup milk for 5 minutes. Add the ground beef, ground pork, chopped onion, minced garlic, egg, parsley, salt, and black pepper. Gently mix with your hands until just combined, being careful not to overwork the meat.
03 - Scoop approximately 2 tablespoons of the meat mixture and flatten it in your palm. Place a mozzarella cube in the center and carefully shape the meat around the cheese, rolling it into a ball to seal the cheese completely inside. Repeat with the remaining mixture.
04 - Arrange the stuffed meatballs on the prepared baking sheet, spacing them evenly. Bake for 20 to 22 minutes, or until cooked through and golden brown on the outside.
05 - While the meatballs bake, melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 to 2 minutes, stirring constantly to form a smooth roux.
06 - Gradually whisk in 1 cup milk, pouring slowly while whisking to prevent lumps. Continue cooking and stirring for 2 to 3 minutes until the mixture thickens to a smooth consistency.
07 - Reduce heat to low. Stir in the grated cheddar, cayenne pepper, smoked paprika, and pickled jalapeños if using. Stir continuously until the cheese has fully melted and the sauce is smooth and creamy. Season with salt and pepper to taste.
08 - Transfer the hot meatballs to a serving platter and drizzle generously with the spicy cheese sauce. Garnish with additional chopped parsley if desired. Serve immediately alongside crusty bread, pasta, or rice.