1. Peel cucumber if desired. Half and seed. Slice cucumber into thin slices (or get thin strips with a potato peeler) and place in a bowl. Half the peaches, remove the pit and slice into ¼ inch slices. Add the sliced peaches, onion and salt to the bowl; toss to combine. Set aside.
2. Heat remaining ingredients in a saucepan until simmering and pour hot liquid over the cucumber-peach mixture. Toss to combine and refrigerate 4 hours or overnight.