Preheat oven to 375°F (190°C).
Bring a large pot of salted water to a boil and cook lasagna noodles according to package directions until al dente; drain and set aside.
In a large skillet over medium-high heat, brown the ground beef with diced onions until meat is no longer pink. Drain excess fat.
Stir in minced garlic and cook for 1 minute until fragrant. Add marinara sauce and simmer for 10 minutes.
In a medium bowl, combine ricotta cheese, egg, 1/4 cup Parmesan cheese, dried oregano, salt, and pepper; mix until well incorporated.
To assemble, spread 1 cup of meat sauce on the bottom of a 9x13 inch baking dish.
Place 3 to 4 noodles over the sauce. Spread one-third of the ricotta mixture over the noodles, followed by 1 cup of mozzarella and 1 cup of meat sauce.
Repeat the layers twice more (noodles, ricotta, mozzarella, sauce).
Top with the remaining mozzarella and the remaining 1/4 cup Parmesan cheese.
Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 20-25 minutes until cheese is bubbly and golden brown.
Let the lasagna rest for 15 minutes before slicing and serving. Garnish with fresh parsley.