Arrange a rack in lowest level of oven; preheat to 300°F. Leaving fat intact, remove outer rind from most of ham, leaving a band around the end of the shank bone. Score fat crosswise (do not cut into meat) on top of ham with parallel cuts spaced 1/2" apart. Place ham in a large roasting pan. Boil 1 cup Riesling and 7 cups water in a saucepan for 5 minutes. Pour into bottom of roasting pan. Bake ham, basting with pan juices occasionally, until an instant-read thermometer inserted into center of ham registers 110°F, 2 1/2-3 hours.
Meanwhile, melt butter in a medium skillet over medium heat. Add shallots and thyme sprigs; cook, stirring often, until shallots are very soft, about 10 minutes. Remove pan from heat; stir in the remaining 1 cup Riesling. Return to stove. Increase heat to medium-high, bring to a simmer, and cook until reduced to 1/4 cup, about 8 minutes. Remove the thyme sprigs and transfer mixture to a food processor. Add thyme leaves, mustard, honey, pepper, and salt. Process until well blended.
Remove pan from oven and increase heat to 350°F. Using a pastry brush, spread Riesling mixture over ham. Return pan to oven and bake ham, tenting with foil if browning too quickly, until internal temperature registers 135°F and crust is golden brown, 15-30 minutes.
Transfer ham to a large platter. Let rest for 30 minutes before carving. Skim fat from pan juices, reheat, and pour juices into a medium pitcher; serve alongside.