Bring a large pot of salted water to a boil. Add the lasagna noodles and cook just until pliable according to package directions. Drain the noodles and set them aside. (Do not stack them atop one another or they will stick together.)
In a large bowl, stir together 4 cups shredded mozzarella cheese with the ricotta cheese, Parmesan cheese, egg and parsley. Set the mixture aside.
Add the ground beef and Italian sausage to a large heavy-bottomed stock pot set over medium-high heat. Cook, breaking the meat apart with a spatula until it is no longer pink, about 5 minutes. Add the onions, garlic and tomato paste and cook, stirring as needed, until the onions are translucent, about 3 minutes. Stir in the marinara sauce. Simmer the sauce for 5 minutes then remove it from the heat.
Add 2 cups of the meat sauce to the bottom of a 13x9-inch baking dish. Arrange 3 lasagna noodles on top. Spread 3/4 cup of the ricotta cheese mixture atop the noodles then repeat the meat sauce, noodles and cheese layers two more times, ending with the meat sauce.
Preheat the oven to 375°F. Cover the baking dish with foil.
Bake the lasagna, covered, for 30 minutes. Remove the foil and sprinkle the remaining 2 cups shredded mozzarella cheese on top. Return the lasagna to the oven and bake, uncovered, until the cheese is golden and bubbling. (If desired, broil the lasagna 3 inches from the top broiler until the cheese is bubbling.)
Allow the lasagna to rest for 15 minutes before slicing and serving topped with additional grated Parmesan cheese.