Boil chicken until cooked through (no longer pink inside), about 20-30 minutes. Chop into bite size pieces and place in a 9x13 inch baking dish.
Combine soup, sour cream, onion (optional) and mushrooms (optional). Pour mixture over chicken and top with crumbled crackers. Cover and bake at 350 degrees F (175 degrees C) for 30 minutes (or freeze for baking at another time).