Place ham in 6-qt. or larger slow cooker sprayed with cooking spray.
Mix jam, sugar and mustard until blended; spoon over ham. Cover with lid.
Cook on low for 3 to 4 hours or until ham is heated through.
Transfer ham to platter, reserving jam mixture in slow cooker, cover ham to keep warm. Turn slow cooker to HIGH. Whisk cornstarch and water until blended; whisk into reserved jam mixture. Cook, uncovered, 10 to 15 min. or until thickened, stirring occasionally.
Spoon half the jam mixture over ham. Serve ham with remaining jam mixture on the side.