Place ham on flat surface. Using a sharp knife, make 1-inch deep cuts diagonally across ham. Turn ham 90 degrees and make cross cuts, creating a diamond pattern. Transfer ham to rack in a large roasting pan. Add 1 cup of water to the bottom of the pan. Cover tightly with foil.
Bake on center rack of oven for 2 to 2-1/2 hours.
In a small bowl, mix brown sugar, rum and mustard until smooth.
Uncover and brush liberally with the glaze mixture, getting into the cuts with the brush. Return to oven, uncovered, for 15 minutes. Remove from oven and apply glaze again. Return to oven for 15 minutes or until glaze is caramelized.