- In a large slow cooker, stir together brown sugar and mustard until sugar has dissolved.
- Place ham in slow cooker and turn, rubbing mustard mixture all over.
- Pour maple syrup over ham. Cover and cook on low until heated through, 4 to 5 hours.
- Remove ham to a cutting board and let rest, loosely covered with foil, for 10 minutes.
- Pour sauce into a serving bowl. Slice ham and serve with sauce on the side.
- Top ham with a dollop of whole-grain mustard, if desired.