Preheat the oven to 375 degrees. Bring a large pot of salted water to a boil and cook the lasagna noodles until al dente. Drain and set aside.

In a large skillet over medium heat, cook the ground beef with the diced onion and minced garlic until the beef is browned and the onion is soft. Drain any excess fat.

Stir 2 cups of the marinara sauce into the beef mixture along with the oregano, salt, and pepper. Simmer for 5 minutes, then remove from heat.

In a mixing bowl, combine the ricotta cheese, egg, and 1/4 cup of the Parmesan cheese until smooth.

Spread 1/4 cup of the remaining marinara sauce in the bottom of a 9x9 baking dish. Lay 3 noodles over the sauce.

Spread half the ricotta mixture over the noodles, followed by half the beef sauce and 2/3 cup mozzarella. Repeat the layers once more with 3 noodles, the remaining ricotta, the remaining beef sauce, and another 2/3 cup mozzarella.

Top with the final 3 noodles, the last 1/4 cup marinara, the remaining mozzarella, and the rest of the Parmesan. Cover with foil and bake for 25 minutes. Remove the foil and bake 10 minutes more until the cheese is bubbly.