Bring a large saucepan filled two-thirds with water to a boil. Add lasagna noodles and cook till al dente. Drain and rinse with cold water. Set aside.
In a Dutch oven or large heavy-duty saucepan/skillet, cook the sausage and beef over medium heat until no longer pink. Drain all but 2 tablespoons of the grease and add the onion. Cook, stirring often, for 5 minutes or until veggies are soft and fragrant. Add the spaghetti sauce and let cook 5 minutes or until warm.
In a medium bowl, combine the Ricotta, 2 cups of the mozzarella, egg and Italian seasoning. Mix well.
Preheat oven to 375 degrees F. Coat a 9×13 baking dish with cooking spray. Layer in even layers as follows: 1/3 of the meat-sauce mixture, 3 cooked noodles, 1/3 of the cheese mixture. Do this three times.
Top with any extra sauce, 1 cup of mozzarella cheese and the Parmesan cheese. Cover with foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 20 minutes. Let sit for 5 minutes before serving.