Bring a large pot of salted water to a boil and cook the lasagna noodles until al dente. Drain them and lay them flat on a tray to prevent sticking.

Heat a large skillet over medium heat. Add the ground beef, chopped onion, and minced garlic. Cook until the meat is browned and the onion is soft. Drain any excess fat.

Pour the marinara sauce into the skillet with the meat mixture. Add the oregano and a pinch of salt and pepper. Let it simmer for 5 minutes to blend the flavors.

In a mixing bowl, stir together the ricotta cheese, egg, 1/4 cup of the Parmesan, and a little salt and pepper until smooth.

Preheat the oven to 375°F. Spread a thin layer of meat sauce in the bottom of an 8x8 baking dish. Add a layer of noodles, then half the ricotta mixture, followed by a third of the mozzarella. Repeat the layers once more.

Finish with the remaining meat sauce, the rest of the mozzarella, and the remaining Parmesan on top.

Bake for 25 to 30 minutes until the cheese is bubbly and lightly golden. Let the lasagna rest for 10 minutes before slicing.