Heat a frying pan over medium heat and brown the chicken on both sides. Place the chicken in a single layer on the bottom of a 9x13 (or larger roasting pan).
Combine the mustard, maple syrup, vinegar, oil, salt and pepper in a small bowl and whisk or stir until combined.
Toss together the sliced onions and sauce until evenly coated.
Pour the onion mixture over the chicken, completely smothering it.
Cover the pan tightly and bake for an hour and a half.
Remove the foil and bake for an additional 30-45 minutes, until cooked through and soft.
If you prefer VERY soft chicken, you can leave it covered the entire time.
Serve hot.
This chicken freezes nicely.
Recipe by Overtime Cook at https://overtimecook.com/2014/02/26/maple-mustard-roasted-chicken/