Preheat the Oven: Preheat your oven to 375°F.
Prepare the Sauce: In a large pot, heat the olive oil over medium heat. Add the chopped onion and minced garlic. Sauté until the onion is soft and translucent. Add the ground beef and cook until browned, breaking it up as it cooks. Season with salt, pepper, and Italian seasoning. Stir in the marinara sauce and let the mixture simmer for about 10 minutes.
Prepare the Noodles: While the sauce simmers, cook the lasagna noodles according to the package instructions until al dente. Drain and set aside.
Mix the Cheese Filling: In a bowl, combine the ricotta cheese, a beaten egg, half of the shredded mozzarella cheese, and all of the grated Parmesan cheese. Mix until smooth.
Assemble the Lasagna: Spread a thin layer of meat sauce over the bottom of a 9×13-inch baking dish. Place a layer of noodles over the sauce. Spread one-third of the cheese mixture evenly over the noodles. Add another layer of meat sauce over the cheese. Repeat the layering process (noodles, cheese mixture, and meat sauce) until all ingredients are used, finishing with a top layer of meat sauce.
Top with Cheese: Sprinkle the remaining shredded mozzarella cheese evenly over the top layer.
Bake: Cover the baking dish with aluminum foil (ensuring the foil does not touch the cheese). Bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Rest and Serve: Remove the lasagna from the oven and let it rest for 10 minutes before cutting. Garnish with chopped fresh basil if desired, then serve.