Preheat the oven: Preheat your oven to 375°F and lightly grease a 9x13-inch baking dish with olive oil or cooking spray.

Brown the ground beef: Heat olive oil in a large skillet over medium heat and cook the onion for 3 minutes until softened. Add the ground beef and garlic, breaking up the meat with a spoon, and cook until no pink remains, about 7 minutes.

Simmer the meat sauce: Stir in the marinara sauce, Italian seasoning, salt, and pepper. Reduce heat to low and simmer uncovered for 10 minutes, stirring occasionally, until slightly thickened.

Boil the noodles: Cook the lasagna noodles in salted boiling water just until bendable, about 8 minutes. Drain and lay them flat on a baking sheet so they don't stick together.

Mix the ricotta filling: In a medium bowl, stir together the ricotta cheese, egg, and Parmesan cheese until smooth and fully combined.

Spread the first sauce layer: Spoon a thin layer of meat sauce over the bottom of the prepared baking dish to keep the noodles from sticking.

Layer the noodles and filling: Place 3 noodles over the sauce, then spread a third of the ricotta mixture over them, followed by a third of the remaining meat sauce and a cup of mozzarella.

Repeat the layers: Repeat the noodle, ricotta, sauce, and mozzarella layers two more times, finishing with a generous layer of mozzarella on top.

Bake covered: Cover the dish tightly with foil and bake for 30 minutes so the noodles finish cooking through without the cheese drying out.

Bake uncovered until golden: Remove the foil and bake for another 15 minutes, until the cheese turns golden and bubbly at the edges.

Rest before slicing: Let the lasagna rest for 10 minutes before cutting so the layers set and hold their shape.

Garnish and serve: Sprinkle chopped parsley over the top and slice into squares to serve.