How to Make It
Preheat oven to 350°F. Line a roasting pan with heavy-duty aluminum foil. Drape several long sheets of foil over pan; place ham, flat side down, in center of foil sheets. Stir together brown sugar, whole-grain mustard, and Dijon mustard in a small bowl; rub evenly over outside of ham, pushing in between slices. Lift sheets up, and tightly wrap ham in a pouch, leaving top of pouch open. Gently pour grape juice over ham inside foil. (Juice should not leak from the foil pouch.) Using additional foil, completely seal ham. Pour 2 cups water into roasting pan.
Bake in preheated oven until a meat thermometer inserted into thickest portion registers 120°F, about 3 hours, 15 minutes. (Partially unwrap foil so you don’t lose the drippings before inserting thermometer.) Remove ham from oven.
Increase oven temperature to 500°F. Let ham stand until cool enough to handle, about 10 minutes. Transfer ham to a large platter, keeping drippings inside foil pouch. Pour drippings (about 4 cups) into a large skillet, discarding foil pouch. Stir in honey, and bring to a boil over high. Reduce heat to medium-high, and cook, stirring occasionally, until thickened and reduced to about 1 1/2 cups, 15 to 20 minutes. Drain water from foil-lined pan; return ham to pan, and brush with 1/2 cup of the glaze.
Return ham to oven, and bake at 500°F until top is caramelized and a meat thermometer inserted into thickest portion registers 140°F, 15 to 20 minutes, brushing with 1/2 cup glaze halfway through. Let stand 15 minutes, and serve with remaining 1/2 cup glaze.