Cook chicken, prepare spaghetti noodles according to package. In a sauce pan, combine Cheez Whiz, cream of mushroom soup and milk. Mix and cook on medium heat until hot. Mix in chicken, spaghetti noodles, onion into sauce mixture. Stir until everything is mixed together.
Pour into a 9 x 13 pan and sprinkle cheese over the top.
Put into oven at 350 degrees F until cheese is melted.