Using your hands, gently mix together the pork, beef, breadcrumbs, Parmesan cheese, salt, eggs, garlic, and parsley in a large bowl. Form the mixture into 2-inch (5 cm) balls.
Heat the oil in a large oven-safe skillet over medium-high heat. Add the meatballs and cook, turning them until browned all over, about 10 minutes. Use a slotted spoon to transfer them to a plate.
Reduce the heat to medium. Add the onion and pepper flakes; cook, stirring occasionally, until the onion is tender. Add the tomatoes and basil; simmer, stirring frequently, until the sauce is slightly thickened. Return the meatballs and their juices to the pan. Simmer, turning the meatballs, until they are cooked through.
Preheat the broiler and arrange the mozzarella cheese on top of the meatballs. Broil until melted, about 2 minutes. Serve immediately.