One of the last stone fruits of the season, Italian prune plums are typically available from mid-August to September.
Bring sugar, water, peppercorns, and a pinch of salt to a boil in a saucepan. Add plums; return to a boil. Let cool. Refrigerate until ready to use (up to 1 week). Serve over ice, topped with seltzer or a combination of seltzer and red wine.