1. Using your hands, gently mix together pork, beef, breadcrumbs, pecorino, salt, eggs, garlic, and parsley in a large bowl. Form mixture into 2-inch balls.

2. Heat oil in a large ovenproof skillet over medium-high heat. Add meatballs; cook, turning, until browned all over, about 10 minutes. Use a slotted spoon to transfer to a plate.

3. Reduce heat to medium. Add onion and pepper flakes; cook, stirring occasionally, until onion is tender, about 4 minutes. Add tomatoes and basil; simmer, stirring frequently, until sauce is slightly thickened, about 8 minutes. Return meatballs and any juices to pan. Simmer, turning meatballs once, until cooked through, about 20 minutes.

4. Heat broiler. Arrange mozzarella on top of meatballs. Broil until melted, 2 to 3 minutes; serve.