Lightly spray a 13 x 9 casserole dish with cooking spray and arrange onion slices over the bottom. Arrange the chicken legs in a single layer on top of the onions, and then lightly season the chicken with the pepper.
In a medium size, bowl mixes together the maple syrup, vinegar, Dijon mustard, and salt. Distribute the sauce evenly over the top of the chicken and bake uncovered for 50 – 60 minutes or until chicken is nicely browned and reaches an internal temperature of 180F with an instant-read thermometer.