Chop onion

Trim chicken and pound between two sheets of plastic wrap to an even thickness of 1/4 inch

Melt 2 tablespoons butter in a large skillet over medium high heat

Add the chicken and brown 2 minutes on each side

Remove the chicken to a plate and set aside. Add the remaining butter to the skillet and cook the onion until tender

Add the ketchup, brown sugar, vinegar, mustard and cranberries

Cook, stirring often, until cranberries pop and sauce thickens, about 8 minutes

Return chicken to the skillet and coat well in the sauce. Cover and simmer over low heat for 8 to 10 minutes or until the chicken is cooked through.