Cut an X through shell on flat side of each chestnut with a small, sharp knife. Simmer chestnuts in water, covered, until tender and sweet (peel one to check), 18 to 20 minutes.
Remove from heat and leave in hot water. Take out a few at a time; let stand just until cool enough to touch. Pull off outer shell and inner brown skin with fingers or a knife.
Microwave honey with pickling spice until boiling; let cool.
Top baguette slices with cheese, watercress, and chestnuts (save extras for another use). Rewarm honey slightly; discard large, round allspice. Spoon honey with remaining spices over crostini. Add salt and pepper to taste. Serve with plenty of napkins.