Preheat the oven to 350 degrees F.

Cook the spaghetti noodles according to package instructions until al dente. Drain and set aside.

In a large skillet, heat olive oil over medium heat. Add diced onion and minced garlic; sauté until soft and fragrant, about 3-5 minutes.

Add the ground beef, breaking it apart, and cook until browned, about 7-10 minutes.

Once the meat is browned, stir in the Italian seasoning, salt, and black pepper. Mix well before adding the marinara sauce and diced tomatoes. Let it simmer for 5 minutes.

In a large bowl, combine the cooked spaghetti noodles with the meat sauce mixture. Stir in the chopped parsley and one half of the Parmesan cheese.

Coat a 13- x 9-inch baking dish with nonstick cooking spray. Spoon half of the spaghetti mixture into the dish and spread it evenly.

Add whisked egg and ricotta cheese in dollops on top. Layer with the remaining spaghetti.

Sprinkle the remaining mozzarella and Parmesan cheeses over the top of the casserole.

Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and continue to bake for an additional 15-20 minutes, or until the cheese is golden and bubbly.