1. Break spaghetti in half. Cook spaghetti according to package directions just until tender. Drain; transfer spaghetti to a large mixing bowl.
2. Meanwhile, in a large skillet cook chicken, mushrooms, onion, and garlic in hot oil over medium-high heat for 3 to 4 minutes or until chicken is cooked through. Add chicken mixture to spaghetti.
3. For sauce, in a small saucepan melt 1 tablespoon margarine or butter. Stir in flour and pepper. Add milk all at once. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. Add American cheese, stirring until melted. Remove from heat. Stir in olives, parsley, and pimiento. Pour the sauce over the chicken mixture. Toss to coat.
4. Transfer to a greased 2-quart square baking dish or casserole. In a small mixing bowl toss together bread crumbs, parmesan cheese, and 1 tablespoon melted margarine or butter. Sprinkle over chicken mixture.
5. Cover and bake in a 375 degree F oven for 20 minutes. Uncover and bake 10 to 15 minutes more or until heated through.