Preheat oven to 300 degrees F (150 degrees C).

Place ham into a roasting pan. Spread Dijon mustard over top and sides of ham in an even coat; sprinkle ham with brown sugar. Press brown sugar onto ham with your hands. Place pineapple slices onto ham, securing them to the top and sides with cloves. Place any unused pineapple slices into bottom of roasting pan and pour pineapple juice and half the orange-flavored rum.

Bake in the preheated oven until ham is heated through and brown sugar has begun to glaze onto the meat, about 1 hour. Remove ham from oven and pour remaining 2 fluid ounces of orange rum over the ham; carefully light the rum on fire with a match. The flames will help the glaze to adhere to the meat.

Continue baking until an instant-read meat thermometer inserted into the thickest part of the ham, not touching bone, reads at least 140 degrees F (60 degrees C), about 1 more hour. Let ham rest for 10 minutes before serving. Transfer ham to a serving platter, leaving pan drippings in roasting pan.

Stir cornstarch and water together in a small bowl until smooth; whisk cornstarch into pan drippings. Place roasting pan over medium-low heat and whisk until pan juices thicken, about 5 minutes. Serve with ham.