Cook spaghetti according to package directions. Meanwhile, in a large skillet, brown beef and onion; drain. Stir in the spaghetti sauce; set aside.
In a large bowl, whisk the eggs, Parmesan cheese and butter. Drain spaghetti; add to egg mixture and toss to coat.
Place half of the spaghetti mixture in a greased 13-in. x 9-in. x 2-in. baking dish. Top with half of the (cottage cheese), meat sauce and mozzarella cheese. Repeat layers. Cover and bake at 350 degrees for 30 minutes. Uncover; bake 15-20 minutes longer or until cheese is melted. (times may vary with your oven).
* Note: if you use cottage cheese you will want to add 5-10 minutes on your baking time.