Add the mustard, tamari, honey and ginger to a 9-by-13-inch baking dish. Whisk well to combine. Add the chicken pieces and turn to coat them evenly in the marinade, ending with the skin-side up. Cover with plastic wrap and refrigerate for at least 2 hours and up to 8.
Remove the chicken dish from the refrigerator 30 minutes before roasting. Toss the chicken pieces again to coat well.
Preheat the oven to 400 degrees F.
Add 1/4 cup water to the baking dish and swirl to combine. Roast, basting every 15 minutes, until the internal temperature of the chicken reaches 160 degrees F on a digital thermometer, 45 to 50 minutes. Allow the chicken to rest for 10 minutes before serving. Serve with the reduced sauce.