Cook pasta according to package directions. Preheat oven to 350. In large skillet, over medium heat, cook celery and onion in margarine until tender. Stir in flour and boullion. Gradually stir in milk. Cook and stir until thickened and bubbly. Remove from heat. Add cheese; stir until melted. Stir in pasta, chicken, and pimento. Spoon into a greased 9x13 baking dish. Cover. Bake 30 minutes or until hot and bubbly. Uncover; top with French fried onions, if desired. Bake 5 minutes longer. Let stand 10 minutes.