In a large mixing bowl, combine ground pork and ground beef. Stir breadcrumbs into meat and then pour milk on top. Let the milk absorb into the meat and breadcrumbs while you get the onions ready.
Stir onion, egg yolk, freshly grated Parmesan cheese, Italian seasoning, salt, and pepper into the meat and breadcrumb mixture.
Shape meatballs and bake the meatballs in the oven at 425°F for 20 minutes.
Combine drained pasta and 14 of the juicy meatballs in a large mixing bowl.
Cover with marinara sauce and stir until both the pasta and tender meatballs are completely covered in sauce. Transfer into casserole dish and spread Greek yogurt over the top, stirring slightly to incorporate it.
Sprinkle mozzarella cheese over the casserole. Bake in a 375°F oven for 25 to 30 minutes until the cheese is melted and the sauce bubbles.
Garnish with fresh basil and a few tablespoons of freshly grated Parmesan cheese. Serve immediately.