Stir together 1/4 cup mustard, the apple juice, rosemary, soy sauce, and garlic. Insert meat thermometer near the center of meat.
Arrange preheated coals around a drip pan in a covered grill. Test for medium-low heat above pan. Place meat on grill over drip pan. Cover and grill for 1-1/2 to 2 hours or until thermometer registers 160 degrees F for medium to 170 degrees F for well doneness, brushing occasionally with mustard mixture up to the last 5 minutes of grilling. Remove meat from grill and cover with foil. Let stand for 15 minutes before slicing.
Meanwhile, toast cut sides of rolls on grill. Remove strings from lamb; slice thinly. Serve on kaiser rolls. Pass additional mustard, if desired. Makes 8 servings.