Cook the noodles.
Bring a large pot of salted water to a boil. Cook the lasagna noodles for the minimum package time, then drain and rinse with cold water to stop the cooking. Lay them out so they do not stick together while you prep the rest.
Start the sauce.
Heat the olive oil in a large skillet over medium-high heat. Add the chopped onion and cook until softened. Stir in the ground beef and cook until browned, breaking it up as it cooks. Drain any excess grease, then season with salt and pepper. Add the garlic and cook briefly, just until fragrant.
Simmer the meat sauce.
Stir in the marinara, Italian seasoning, and parsley. Bring the mixture to a gentle simmer, then cook on low for 10 to 15 minutes so the flavors blend.
Mix the ricotta layer.
In a medium bowl, beat the egg lightly, then stir in the ricotta until smooth and combined. Shred the mozzarella if needed.
Assemble the lasagna.
Spread a little meat sauce in the bottom of a 9x13-inch baking dish. Add a layer of 3 noodles. Spoon over part of the remaining sauce, then add half of the ricotta mixture, followed by part of the mozzarella and Parmesan. Repeat the middle layer once more. Finish with the last 3 noodles, the remaining sauce, and the rest of the cheese on top.
Bake.
Cover the dish with foil and bake at 375°F for 35 minutes. Remove the foil and continue baking for 10 to 15 minutes, until the lasagna is bubbling and the cheese is lightly golden.
Rest before serving.
Let the lasagna stand for about 10 minutes before slicing. That short rest helps the layers hold together much better