In a greased 4-qt. slow cooker, combine the first seven ingredients. Cook, covered, on low 3-4 hours or until heated through.
Preheat broiler. Place bun bottoms and tops on baking sheets, cut side up. Using a slotted spoon, place ham mixture on bottoms; top with cheese. Broil 3-4 in. from heat 1-2 minutes or until cheese is melted and tops are toasted. Replace tops. If desired, serve with additional mustard.
Adapted from: www.tasteofhome.com