Chicken Tetrazzini

1 chicken breast, cooked and cubed
1 (10 oz) pkg spaghetti, broken
1/2 c margarine
1 small onion, diced
1 lb velveeta
1 can cream of mushroom soup

Cook spaghetti until done.  Drain.  Melt margarine and saute onion.   Add cheese, stirring until  melted.  Add to spaghetti.  Add soup and chicken.  Put in baking dish and bake for 30 minutes at 350.