Preheat the oven to 375 degrees. Cook the lasagna noodles according to package directions, then drain and set them aside.
In a large skillet over medium heat, cook the ground beef with the chopped onion and minced garlic until the beef browns. Drain any excess fat.
Stir the tomato sauce, oregano, salt, and pepper into the beef mixture. Simmer for 5 minutes to blend the flavors.
In a mixing bowl, combine the ricotta cheese with the egg until smooth.
Spread a thin layer of the beef sauce in the bottom of a baking dish. Add a layer of noodles, then half the ricotta mixture, followed by a third of the mozzarella and Parmesan.
Repeat the layers once more, ending with a final layer of noodles, the remaining beef sauce, and the rest of the mozzarella and Parmesan on top.
Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for 10 more minutes until the cheese turns golden.