Preheat the oven to 400°F (200°C). Grease a 9x13-inch baking dish with olive oil.

Finely mince the onion and gather all ingredients for mixing.

In a large mixing bowl, combine ground beef, bread crumbs, milk, salt, Worcestershire sauce, pepper, minced onion, parsley, egg, and garlic powder. Mix gently until well combined.

Shape meat mixture into 1-inch meatballs, aiming for about 20 to 24. Arrange them in the greased baking dish and bake for 18 to 20 minutes.

While meatballs are baking, cook spaghetti according to package instructions until 1-2 minutes before al dente. Toss with olive oil after draining.

In a separate bowl, whisk together the additional egg, parsley, mozzarella, Parmesan, and pepper. Pour this over the cooked spaghetti and stir gently.

Spread the cheesy spaghetti mixture in the baking dish, layer the meatballs on top, and pour pasta sauce over everything.

Cover dish with aluminum foil and bake at 350°F (175°C) for 35 minutes.

Remove foil, sprinkle remaining mozzarella and Parmesan on top, and bake uncovered for an additional 10 minutes.

Let casserole rest for about 5 minutes before serving.