Preheat the oven to 325F degrees.
Place the pineapple, mustard, and brown sugar in a saucepan, stir to combine, and cook over medium-low heat until the pineapple begins to soften and the brown sugar dissolves, about 5 minutes.
Place the ham in a 13x9 inch pan and coat with the pineapple sauce.
Roast the ham, basting with the pineapple sauce every 30 minutes, until warmed through, about 1 1/2-2 hours.
Slice the ham and serve topped with sauce from the roasting pan.