1. Cut ham into 3/4-inch cubes. Carefully break cheese into 60 small pieces. Cut apple slices into thirds.
2. Thread cheese, apple, watercress leaves, and ham onto skewers. Stand skewers upright, ham ends down, on a serving plate.
Assemble up to one hour ahead, cover with damp paper towels, and chill. Serve coarse-grain mustard or chutney alongside the skewers.