Stir together 1/4 cup mustard, 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 till thermometer registers 160° for medium to 170° 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.