Preheat the oven to 375°.
In a large skillet, cook the ground beef and onion together until the beef is browned and the onion is soft. Drain any excess grease if necessary. Add the minced garlic and cook for another minute. Stir in the marinara sauce and let everything simmer together for about 5 minutes. Remove the meat sauce from the heat and set it aside.
In a medium bowl, combine the ricotta cheese, egg, shredded mozzarella, Parmesan cheese, and oregano until everything is evenly incorporated.
Grease a 9×13" baking dish and spread a thin layer of the meat sauce over the bottom. Arrange a layer of cooked lasagna noodles over the sauce. Spread ⅓ of the ricotta mixture evenly over the noodles, followed by more meat sauce, mozzarella, and Parmesan cheese.
Continue repeating the layers until all of the ingredients have been used, finishing with the remaining meat sauce, mozzarella, and Parmesan cheese on top.
Cover the baking dish tightly with aluminum foil and bake on the center rack for 30 minutes. Remove the foil and continue baking for another 25 minutes, or until the cheese is melted and bubbly.
Remove the lasagna from the oven, sprinkle with fresh parsley, and let it rest for at least 15 minutes before slicing and serving.