Preheat the oven to 375°F (190°C)
Cook lasagna noodles according to package directions if needed
Drain the noodles and set aside
Brown ground beef or sausage in a skillet if using
Drain excess fat
Add marinara sauce to the meat and simmer briefly
Mix ricotta cheese with egg, salt, pepper, and parsley if desired
Spread a thin layer of sauce in a baking dish
Add a layer of noodles
Add a layer of ricotta mixture
Add a layer of meat sauce
Add a layer of mozzarella cheese
Repeat the layers until ingredients are used
Finish with sauce and a top layer of mozzarella and Parmesan cheese
Cover with foil
Bake for 25 to 30 minutes
Remove foil and bake for 15 to 20 minutes more
Let rest for 10 to 15 minutes before slicing
Serve warm