Preheat oven to 400°F Combine maple syrup, mustard, Worcestershire sauce, 1/4 teaspoon salt, and cayenne in a small bowl and set aside.
Season the turkey legs with the remaining salt and the fresh-ground pepper.
Place the onions on the bottom of a roasting pan and put the turkey legs on top.
Dot the turkey with butter and roast for 30 minutes.
Brush turkey legs with the prepared sauce and reduce oven temperature to 325°F Continue to roast, basting with the sauce every 20 minutes, until meat is very tender -- about 90 more minutes. Serve hot or at room temperature.