In food processor grind onion, pepper and olives separately. (Ingredients may be chopped very fine if food processor is unavailable.) Mix with cooked spaghetti. Add tomatoes, mushrooms, whipping cream and salt and pepper. Turn into 2 1/2 quart buttered casserole. Sprinkle with cheese and bread crumbs. Dot with butter.
Bake covered 350 degrees 1-1/2 hours (uncover last 1/2 hour).