Cook spaghetti according to directions; drain and set aside. In a large skillet, cook beef and onion until meat is no longer pink; drain. Stir in sauce and salt; set aside. In a large bowl, whisk the eggs, cheese and butter. Add spaghetti to egg mixture and toss to coat. Place half the spaghetti mixture in a greased 9 x 13 inch baking dish. Top with half the cottage cheese, meat sauce and mozzarella cheese. Repeat layers. Cover and bake at 350F for 40 minutes. Uncover and bake 20-25 minutes longer or until cheese is melted.