Mix meatball ingredients: Using your hands, gently mix pork, beef, breadcrumbs, pecorino, salt, eggs, garlic, and parsley in a large bowl.
Shape meatballs: Form mixture into 2-inch balls.
Brown meatballs: Heat oil in a large ovenproof skillet over medium-high heat. Add meatballs; cook, turning, until browned all over, about 10 minutes.
Remove from skillet: Use a slotted spoon to transfer to a plate.
Add onion and pepper flakes: Reduce heat to medium. Add onion and pepper flakes; cook, stirring occasionally, until onion is tender, about 4 minutes.
Add tomatoes and basil: Add tomatoes and basil; simmer, stirring frequently, until sauce is slightly thickened, about 8 minutes.
Return meatballs to pan and simmer: Return meatballs and any juices to pan. Simmer, turning meatballs once, until cooked through, about 20 minutes.
Add mozzarella and broil: Heat broiler. Arrange mozzarella on top of meatballs. Broil until melted, 2 to 3 minutes; serve.
