Preheat oven to 350 degrees. Prepare two or four 9x13 casserole dishes with nonstick cooking spray.
In a large skillet over medium high heat, brown the ground beef with the onion until the meat is crumbled and no longer pink. Drain well and return to pan. Stir in one jar of marinara sauce with enough water to rinse the jar.
Meanwhile, cook the spaghetti noodles according to package directions. Drain and stir in the remaining jar of marinara sauce.
In a large bowl, beat together the cream cheese, cottage cheese, and 1 cup shredded mozzarella cheese.
In the bottom of the casserole dishes, layer spaghetti, cream cheese mixture, and meatsauce. Repeat at least once.
Top the casserole with remaining cheese.
Cover and bake for 30 minutes. Remove the cover and continue baking for another 15 minutes or until the cheese has melted and the edges bubble. (see alternative step 7 below for freezer meal directions)