Preheat oven to 375° F.

Bring a large pot of salted water to a boil. Add the lasagna noodles and cook according to package directions for the minimum cook time. Then, drain the noodles and rinse them with cold water. Lay them out on parchment paper to keep them from sticking together.

Heat the olive oil in a large skillet over medium-high heat. Add the onion and cook until softened. Add the ground beef and cook until browned. Drain off excess grease. Season with salt and pepper. Add the minced garlic and cook for 30 seconds. Add the marinara sauce, Italian seasoning, and parsley to the skillet. Bring to a simmer and then reduce the temperature to low. Cook at a low simmer for 10 to 15 minutes.

Crack the egg into a medium bowl and beat lightly with a fork. Add the ricotta cheese and stir to combine. Shred the mozzarella cheese.

To assemble the lasagna, put about 1 cup of the sauce into a 9x13-inch baking dish. Lay 3 noodles in an even layer over the sauce. Spread ⅓ of the remaining sauce over the noodles. Place small spoonfuls of the ricotta and egg mixture on top of the sauce, using ½ of the ricotta. Sprinkle ⅓ of the mozzarella and ⅓ of the Parmesan over the top.

Repeat the same layer one more time: 3 noodles, ½ of the remaining sauce, the rest of the ricotta, ½ of the remaining mozzarella, and ½ of the remaining Parmesan.

For the top layer of the lasagna, use the remaining 3 noodles, the rest of the sauce, and finally the rest of the mozzarella and Parmesan.

Cover the lasagna with foil and bake for 35 minutes. Then uncover and continue baking for 10 to 15 minutes more.

Let the lasagna rest for 10 minutes before serving.