01 - Preheat oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - In a large mixing bowl, soak breadcrumbs in 1/4 cup milk for 5 minutes. Add 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 the palm of your hand. Place a mozzarella cube in the center and carefully shape the meat around it, sealing the cheese completely inside. Roll into a ball and repeat with the remaining mixture.
04 - Arrange the stuffed meatballs on the prepared baking sheet, spacing them evenly. Bake for 20 to 22 minutes until fully cooked through and golden brown on the outside.
05 - While the meatballs bake, melt butter in a medium saucepan over medium heat. Whisk in the flour and cook, stirring constantly, for 1 to 2 minutes to form a light roux.
06 - Gradually pour in 1 cup milk while whisking continuously to prevent lumps. Continue cooking and stirring until the mixture thickens, about 2 to 3 minutes.
07 - Reduce heat to low. Stir in grated cheddar, cayenne pepper, smoked paprika, and pickled jalapeños if using. Stir until the cheese is completely melted and the sauce is smooth. Season with salt and pepper to taste.
08 - Transfer the hot meatballs to a serving dish and drizzle generously with the spicy cheese sauce. Garnish with additional chopped parsley if desired and serve alongside crusty bread, pasta, or rice.