Easy Barbecued Chicken

1/3 c. chopped onion

3 T. margarine

3/4 c. ketchup

1/2 c. water

1/3 c. vinegar

3 T. brown sugar

1 T. Worcestershire sauce

2 tsp. prepared mustard

1/4 tsp. salt

1/8 tsp. pepper

2 1/2-3 lbs. boneless, skinless chicken breast

Place all the ingredients (except chicken) into a saucepan and simmer for 15 mins. Pour sauce over chicken and cook in crock pot on high for 4 hours.