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. You can lay them out on parchment paper to keep them from sticking together.
Meanwhile, heat the olive oil in a large skillet over medium-high heat. Add the onion and cook until softened, 3 to 4 minutes, stirring occasionally. Add the ground beef and cook, crumbling, until the beef is browned. Add the minced garlic and cook, stirring, for 30 seconds.
Add the marinara sauce, Italian seasoning and parsley to the skillet and stir. Bring to a simmer and then reduce the temperature to low. Cook at a low simmer for 10 to 15 minutes, stirring occasionally.
While the sauce simmers, crack the egg into a medium bowl and beat lightly with a fork. Add the ricotta cheese and stir to combine. This is also a good time to 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 a piece of foil that has been sprayed with cooking spray on the underside (to prevent the cheese from sticking to the foil).
Bake the lasagna, covered with foil, for 35 minutes. Then uncover and continue baking for 10 to 15 minutes more, or until the lasagna is hot and bubbling on the sides and the cheese is lightly browned on top. Let the lasagna rest for 10 minutes before serving.