Preheat your oven to 400°F and lightly oil a 9x13 baking dish.

In a mixing bowl, combine ground beef, breadcrumbs, milk, salt, Worcestershire sauce, pepper, minced onion, parsley, garlic powder, and egg; mix gently until just combined.

Shape the mixture into 1-inch meatballs and bake on a lined baking sheet for 18-20 minutes until golden brown.

Lower oven temperature to 350°F. Meanwhile, cook spaghetti until al dente, drain, and toss with olive oil.

In a small bowl, whisk together the remaining egg, parsley, ¾ cup mozzarella, and ½ cup parmesan until fully blended.

Gently mix the egg and cheese mixture with the cooked spaghetti and spread evenly in the prepared baking dish.

Nestle the baked meatballs over the spaghetti and pour your favorite pasta sauce all over it.

Cover with foil and bake for 35 minutes; remove foil, sprinkle remaining cheese, and bake uncovered for another 10 minutes.

Let the casserole rest for about 5 minutes before serving.