Ro*Tel Chicken Casserole

1 can Ro*Tel tomatoes with chilies
1 can cream of mushroom soup
1 small container Velveeta cheese, diced
1 small onion, saut ed
1 stick margarine
1 whole chicken
1 cup cheddar cheese
1 package spaghetti

Boil chicken and remove meat from bone. In the same water, boil spaghetti and drain, but do not rinse. Mix all ingredients together, except Cheddar cheese. Bake at 350 degrees F until Velveeta cheese is melted and hot. Garnish with Cheddar cheese and serve.