In a large skillet, cook the beef, green pepper, onion, salt and pepper over medium-high heat for 10-12 minutes or until meat is no longer pink. Add garlic; cook 1 minute longer. Drain. Stir in the spaghetti, mushrooms and olives.
Transfer to two greased 8-in. square baking dishes. Sprinkle with cheeses. Cover and freeze one casserole for up to 3 months. Bake remaining casserole, uncovered, at 350° for 25-30 minutes or until bubbly and golden brown.
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).