How to Make It
Preheat the oven to 325° and position a rack in the bottom third of the oven. Set the smoked ham in a large roasting pan. Score a 1/4-inch-deep crosshatch pattern into the fat at 2-inch intervals. Pour 2 cups of the Dr Pepper and the 2 cups of water into the pan and roast the ham for about 2 1/2 hours, until an instant-read thermometer inserted in the thickest part of the ham registers 120°. Increase the oven temperature to 425°.
Meanwhile, in a medium saucepan, simmer the prunes in the remaining 1 cup of Dr Pepper until they are plump and the liquid is slightly reduced, about 10 minutes. Using a slotted spoon, transfer the prunes to a small bowl and cover with plastic wrap. Whisk the mustard, brown sugar and vinegar into the liquid in the saucepan and boil until very thick and syrupy, about 5 minutes.
Drizzle the syrupy glaze over the ham and roast until glossy, 20 minutes. Carefully transfer the ham to a cutting board.
Pour the pan juices into the saucepan and spoon off the fat. Boil the sauce until reduced to 2 cups, 10 minutes. Whisk in the cornstarch slurry and the prunes and bring to a boil. Simmer the sauce until thickened, 2 minutes. Slice the ham and serve with the Dr Pepper sauce.