Cook pasta, drain and place in a 9x13 inch baking dish.
Saute onion in butter until soft in a large saucepan. Remove from heat; blend in flour, bouillon, salt, dry mustard and pepper.
Place pan on med. high heat and slowly stir in evap. milk and water. Cook, stirring until thickened and the sauce boils for 1 minute. Turn off heat and add pimentos.
Mix 2 cups of the sauce with the drained spaghetti in the baking dish, making a well in the center to hold the chicken. Combine chicken with remaining sauce; spoon into speghetti; sprinkle cheeses on top.
Bake 450 degrees for 20 minutes. The recipe calls for a can of mushrooms. If you use the mushrooms, include the liquid from the can and reduce the amount of water by that much. Sometimes I add a bag of frozen peas to make it a one-dish supper! I increase the cooking time by 10 minutes.