Preheat the oven to 350 degrees with the rack in the middle.
In a small bowl, mix together the jam and mustard. Set aside.
Pour the pineapple in to a large baking dish and spread it around the pan. If using slices of pineapple, just layer them on the bottom. (I use a deep ceramic 9x13 baking dish, or 15x10 depending on the size of the ham.)
Place the ham on top of the pineapple, fatty side up..
Spoon the jam and mustard mixture all over the top of the ham, and spread it gently to the edges of the ham.
Cover with a large piece of foil and place in the oven for about one hour for a 4-5 pound ham. (Add 15 minutes for each additional pound.)
Remove the foil, turn the oven up to 425 and let the ham bake uncovered for 15-20 minutes until it's nice and brown, caramelized, and even a little crispy in places.
Remove from the oven and let sit for about 10 minutes before slicing.
To slice, cut the whole ham in half from front to back, then slice each half in thin slices. Transfer to serving platter and garnish with pineapple. Spoon some of the juices over the top if desired!