In a large skillet, cook beef and onion over medium heat until meat is no longer pink, breaking meat into crumbles; drain. Sprinkle with salt and 1/4 teaspoon pepper; set aside. , In a large bowl, combine the pasta, 3 cups mozzarella cheese, cottage cheese, eggs, Parmesan cheese, parsley and remaining pepper. Transfer to a greased shallow 3-qt. or 13x9-in. baking dish. Top with beef mixture and spaghetti sauce (dish will be full). , Cover and bake at 350° for 45 minutes. Sprinkle with remaining mozzarella cheese. Bake, uncovered, until bubbly and cheese is melted, 15 minutes longer. Let stand for 10 minutes before serving. Freeze option: Sprinkle casserole with remaining mozzarella cheese. Cover and freeze unbaked casserole. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Bake casserole as directed, increasing time as necessary until heated through and a thermometer inserted in center reads 165°.