In a bowl, mix the dry mustard with the coriander, curry powder, brown sugar and salt. Spread the lamb legs on large baking sheets and rub them all over with the spice mix. Cover with plastic wrap and refrigerate for at least 6 hours or overnight.
Light a grill. Lightly season the lamb legs with salt and pepper and grill over moderate heat until nicely charred and an instant-read thermometer inserted in the thickest part of the meat registers 130°, about 20 minutes per side.
During the last 3 minutes of grilling, brush the lamb all over with the hoisin sauce. Transfer the lamb to a carving board and let rest for 10 minutes. Slice the lamb across the grain, arrange on a platter and serve.