Position an oven rack in the lower third of the oven. Preheat the oven to 400 degrees Fahrenheit. Line a rimmed baking sheet with foil or parchment paper.
In a large bowl, combine the onion, parsley, parmesan, bread crumbs, egg, ketchup, garlic, red pepper flakes, salt, pepper. Add the beef and turkey. Combine all ingredients gently but thoroughly.
Shape the meat mixture into 16 meatballs. They should be about 1-1/2 inch in diameter each.
Make a hole in the center of each meatball and place a cube of mozzarella inside. Reshape the meatball so that the mozzarella is completely covered with the meat mixture.
Bake the meatballs for 15 minutes until cooked through.
While the meatballs bake, bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally. This should take about 8 -10 minutes.
Drain the pasta and place in a large serving bowl. Add the meatballs and sauce. Toss gently and season with salt and pepper, to taste. Liberally garnish with parsley and parmesan.