Preheat oven to 375°F (190°C).
Cook lasagna noodles in boiling salted water according to package directions until al dente; drain and set aside.
In a large skillet, brown ground beef with onions and garlic over medium heat until fully cooked; drain excess fat.
Stir marinara sauce into the meat mixture and simmer for 10 minutes.
In a medium bowl, combine ricotta cheese, egg, half of the parmesan cheese, parsley, salt, and pepper.
Spread 1 cup of meat sauce in the bottom of a 9x13 inch baking dish.
Layer with 3-4 noodles, 1/3 of the ricotta mixture, 1/3 of the mozzarella cheese, and 1 cup of meat sauce.
Repeat layers twice more, finishing with a final layer of mozzarella and the remaining parmesan cheese.
Cover with foil and bake for 25 minutes.
Remove foil and bake for an additional 25 minutes until cheese is bubbly and golden.
Let rest for 15 minutes before slicing and serving.