Preheat the oven to 375 degrees and bring a large pot of salted water to a boil. Cook the lasagna noodles according to package directions until al dente, then drain and set aside.

Heat the olive oil in a large skillet over medium heat. Add the ground beef, Italian sausage, and diced onion. Cook until the meats are browned and the onion is soft, breaking up the meat as it cooks.

Stir in the minced garlic, Italian seasoning, salt, and black pepper. Cook for one minute more, then add the marinara sauce. Simmer the mixture for 5 minutes to let the flavors combine.

In a small bowl, mix the ricotta cheese with half of the Parmesan. Spread a thin layer of the meat sauce in the bottom of an 8x8 baking dish.

Layer three noodles over the sauce, followed by half of the remaining meat sauce, half of the ricotta mixture, and one third of the mozzarella. Repeat the layers once more, ending with the last three noodles, the remaining meat sauce, and the rest of the mozzarella and Parmesan on top.

Cover the dish with foil and bake for 20 minutes. Remove the foil and bake for another 10 minutes until the cheese is bubbly and lightly golden. Let the lasagna rest for 10 minutes before slicing.