Preheat the oven to 375°F (190°C) and grease a 9×13-inch baking dish.

Cook the spaghetti in salted boiling water until al dente, then drain.

In a large bowl, mix the cooked spaghetti with egg, milk, half of the mozzarella and parmesan, garlic powder, Italian seasoning, salt, and pepper.

Spread half of the marinara sauce on the bottom of the baking dish.

Add the pasta mixture and press it gently into an even layer.

Top with the cooked meatballs and pour the remaining marinara sauce over them.

Sprinkle the rest of the mozzarella and parmesan cheese on top.

Cover loosely with foil and bake for 25 minutes.

Remove the foil and bake another 10–15 minutes until the cheese is golden and bubbly.

Let rest for 5–10 minutes before serving.