Preheat your oven to 350°F (175°C). Cook spaghetti according to the package instructions until al dente. Drain and return to the pot. Toss with olive oil to prevent sticking.

In a large skillet over medium heat, cook the onion and garlic until softened, about 3-4 minutes. Add the ground beef, breaking it apart with a spoon, and cook until browned. Drain any excess fat.

Stir the marinara sauce and Italian seasoning into the skillet with the beef. Season with salt and pepper to taste. Simmer for about 10 minutes.

Add the cooked spaghetti to the sauce and toss to combine. Transfer half of the spaghetti mixture to a greased 9x13-inch baking dish.

Sprinkle half of the Parmesan and mozzarella cheeses over the spaghetti in the dish. Repeat layers with the remaining spaghetti mixture and cheeses.

In a small bowl, whisk together the egg and milk. Pour this mixture evenly over the top layer of spaghetti.

Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and lightly browned.

Let the baked spaghetti stand for a few minutes before serving. Garnish with chopped fresh parsley if desired.