Heat a large skillet over medium heat. Add the ground beef and Italian sausage, cooking until browned (7 to 10 minutes). Drain excess fat.
Add diced onion and minced garlic to the skillet. Sauté for about 5 minutes until the onion is translucent.
Stir in crushed tomatoes, tomato paste, oregano, basil, salt, and black pepper. Simmer for 10 minutes.
In a mixing bowl, combine ricotta cheese, 1 cup mozzarella cheese, Parmesan cheese, and egg. Mix until well combined.
In a 9x13 inch baking dish, spread a layer of meat sauce, then half of the cheese mixture. Repeat layers, finishing with meat sauce and remaining mozzarella cheese on top.
Preheat oven to 375°F. Cover the baking dish with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 15 minutes.
Let the lasagna cool for about 10 minutes before serving.