Place Burgers on grill over medium, ash-covered coals. Grill, uncovered, 13 to 15 minutes (over medium heat on preheated gas grill, covered, 13 to 14 minutes) until instant-read thermometer inserted horizontally into center registers 160°F, turning occasionally.
Meanwhile combine sauce ingredients in small bowl; set aside.
Spread sauce on cut sides of rolls. Place a burger on bottom half of each roll; top evenly with watercress. Close sandwiches.