Using your hands, gently mix together pork, beef, breadcrumbs, parmesan, salt, eggs, garlic, and parsley in a large bowl.
Form mixture into 2-inch balls.
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.
Reduce heat to medium.
Add onion and pepper flakes; cook, stirring occasionally, until onion is tender, about 4 minutes.
Add marinara sauce 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.
Heat Oven 375 F. Degrees.
Arrange mozzarella on top of meatballs.
Broil until melted, 2 to 3 minutes; serve.
Or cover skillet and lower heat and cook until cheese is melted.