How to Make It
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.