Bring a large pot of salted water to a boil and cook the lasagna noodles until just al dente. Drain and set aside on a lightly oiled tray to prevent sticking.

Heat a large skillet over medium heat. Add the ground beef and cook until browned, breaking it up with a spoon. Stir in the diced onion and minced garlic and cook until the onion softens.

Pour the tomato sauce into the skillet. Add the dried oregano, salt, and pepper. Simmer for 5 minutes so the flavors combine, then remove from heat.

In a mixing bowl, stir together the ricotta cheese, egg, and half of the grated parmesan until smooth.

Spread a thin layer of meat sauce in the bottom of a baking dish. Arrange a layer of noodles over the sauce, then spread half the ricotta mixture on top. Sprinkle with some mozzarella. Repeat the layers once more, ending with a final layer of noodles and the remaining meat sauce.

Top with the rest of the mozzarella and parmesan. Cover the dish with foil and bake at 375°F for 25 minutes. Remove the foil and bake 10 minutes more until the cheese is bubbly and lightly browned.

Let the lasagna rest for 10 minutes before slicing. Garnish with fresh parsley just before serving.