Preheat oven to 350 degrees F.
For Ham: Remove any thick skin from ham and all but about 3/4-inch layer of fat.
Mince and mash garlic with salt to a paste. Coarsely grind coriander and peppercorns in coffee/spice grinder and stir together with flour, butter, and garlic paste in a bowl.
Spread spice paste all over ham, then put ham, cut side down, in a greased large roasting pan. Bake in middle of oven until crust is golden and an instant-read thermometer inserted into center (do not touch bone) registers 150 degrees F, about 1 1/4 hours.
Transfer ham to a platter and let stand 15 minutes before slicing. Garnish with parsley.
For Sauce: Stir together mustard and maple syrup until combined well. Serve with ham.
Cook's note: Sauce can be made 2 days ahead and chilled, covered. Bring to room temperature before serving.
Garnish: 2 large bunches fresh flat-leaf parsley .
Special equipment: an electric coffee/spice grinder .