Preheat oven to 350°. In a large skillet, cook beef and onion over medium heat 10-12 minutes or until meat is no longer pink; drain. Stir in pasta from the dinner mix, contents of seasoning mix, water, tomatoes, spinach, mushrooms and onions. Bring to a boil.
Reduce heat; cover and simmer 10-13 minutes or until pasta is tender. Stir in cottage cheese and Parmesan cheese.
Transfer to two greased 8-in. square baking dishes. Sprinkle with mozzarella cheese. Cover and freeze one casserole for up to 3 months.
Cover and bake the remaining casserole 15-20 minutes or until bubbly and cheese is melted.
To use frozen casserole: Remove from the freezer 30 minutes before baking (do not thaw). Cover and bake at 350° for 1 hour. Uncover; bake 15-20 minutes longer or until heated through. Yield: 2 casseroles (4 servings each).