Cut the ham into large chunks so they nestle together in the bottom of the pressure cooker. Trim off any large pockets or pieces of fat and discard.
Fill the pressure cooker to cover the ham (without exceeding the max fill of the pressure cooker) with a 2:1 ratio of water to white vinegar.
Close the lid and valve and pressure cook on high for 45 minutes. Let the pressure naturally release. If the ham doesn't shred easily, wipe the lid dry before closing and securing, and cook for 5-10 more minutes on high pressure.
When finished cooking, carefully remove the ham from the pot to a baking dish, discarding the liquid. Let cool until easier to handle and shred into pieces.
In a small bowl, stir together the brown sugar and mustard.
Place half of the ham in a 9X9-inch baking dish (or a similar size). Sprinkle half of the brown sugar mixture over the ham. Layer the rest of the ham on top and sprinkle with the remaining brown sugar mixture.
Cover the dish tightly with foil and bake at 325 degrees for 1 hour. Serve warm or at room temperature.
Notes
Ham: the original sweet baked ham recipe implicitly states a bone-in unsliced ham should be used. For the pressure cooker, it's hard to get a bone-in ham that can be trimmed down to fit, so I usually use boneless hams (that have NOT been sliced) so they can be cut into large pieces.