Place chicken in a 13 x 9 inch baking dish or a large, clean Ziploc bag.
In a small bowl, combine the ketchup, maple syrup, liquid smoke, vinegar, mustard, salt and pepper. Mix well.
Add ketchup mixture to the chicken and stir well to distribute ingredients.
Marinate chicken overnight or at least 4 hours.
Preheat oven to 350 degrees.
First place half of the onion slices on bottom of baking dish, followed by the marinated chicken, then arrange the remaining onion slices on top. (The chicken can also be grilled). If baking in oven, place aluminum foil over chicken so that the chicken does not burn due to the high sugar content. Remove foil last ten minutes of baking.