Position rack in lower third of oven. Place the ham flat side down in a large shallow roasting pan and score a diamond pattern about 1/8-inch deep into any fat. Season with pepper and bake for 1 1/2 hours. 

Meanwhile, in a small saucepan over medium heat, combine the jelly, butter, and thyme, whisking occasionally until the jelly and butter melt together and the mixture comes to a gentle boil, 1 to 2 minutes. Cover and set aside. 

In a medium bowl, combine the grapes and shallots. Set aside. 

Baste the ham with the jelly mixture. Continue baking, basting with the jelly mixture and/or pan juices about every 15 minutes. When the internal ham temperature reaches 120ºF, add the grapes and shallots to the roasting pan, stirring to coat with the pan juices. Continue baking and basting until internal temperature reaches 140ºF, 15 to 18 minutes per pound total cooking time. Remove the ham from the oven, transfer to a cutting board, and let rest 15 to 30 minutes. (If the grapes and shallots aren’t tender yet, return the roasting pan to the oven.) 

Slice enough ham to serve and arrange on plates or a platter. Season the roasted grape, shallot, and pan juice mixture with pepper and spoon some on top of the ham. Serve the remaining grape mixture on the side.