Pre-heat oven to 400 degrees. Place butter and chopped onion in a 10 x 15" roasting pan. In a 400 degree oven, roast onions uncovered for 10-15 minutes, or until golden. Push onions to the side of the roasting pan.
Place chicken in the same roasting pan, but not on the onions. Bake, uncovered for 25 minutes.
While the chicken cooks, prepare the sauce. In a bowl, stir Katchup, brown sugar, vinegar and mustard together. Add cranberries. At the end of the 25 minutes, scoop out the browned onions and add to the sauce.
Spread chicken evenly in the pan and spoon cranberry mixture (sauce) over the chicken. Bake an additional 25 minutes or until chicken is done and sauce is caramelized.