Preheat oven to 375°F (190°C).
Cook lasagna noodles according to package instructions and drain.
Heat olive oil in a skillet and sauté onion until soft.
Add garlic and cook until fragrant.
Add ground beef and cook until browned.
Stir in marinara sauce, salt, pepper, and Italian seasoning.
Simmer the sauce for 10 minutes.
In a bowl, mix ricotta cheese, egg, parsley, and half of the Parmesan.
Spread a layer of meat sauce in the baking dish.
Add a layer of noodles, ricotta mixture, and mozzarella cheese.
Repeat layers until ingredients are used.
Finish with sauce, mozzarella, and remaining Parmesan.
Cover with foil and bake for 30 minutes.
Remove foil and bake for another 15 minutes until bubbly.
Let lasagna rest for 15 minutes before slicing and serving.