Preheat your oven to 375°F (190°C).
In a large skillet over medium heat, cook the ground beef, Italian sausage, and chopped onion until the meat is browned and the onion is translucent. Drain any excess fat.
Stir in the minced garlic, crushed tomatoes, tomato paste, oregano, basil, salt, and pepper. Let simmer for about 20 minutes, stirring occasionally.
While the sauce is simmering, boil the lasagna noodles according to package instructions. Drain and set aside.
In a mixing bowl, combine the ricotta cheese, egg, and half of the Parmesan cheese. Mix well.
To assemble the lasagna, spread a thin layer of meat sauce on the bottom of a 9x13 inch baking dish. Place 4 noodles over the sauce, followed by half of the ricotta mixture, a third of the mozzarella cheese, and a third of the remaining meat sauce. Repeat the layers.
Finish with a top layer of noodles, the remaining sauce, and the rest of the mozzarella and Parmesan cheese.
Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 20 minutes, or until the cheese is bubbly and golden.
Allow the lasagna to cool for about 10-15 minutes before slicing and serving.