Combine onion and cucumber slices in a clean spring-top jar.
Combine the remaining ingredients in a non-reactive saucepan and bring to a boil. Simmer for 4 full minutes to wake up the flavors of the spices.
Slowly pour the hot pickling liquid over the onion and cucumber slice, completely filling the jar. Allow the pickles to cool to room temperature before topping off with any remaining pickling liquid. Refrigerate.
Refrigerate the pickles for a week to ripen. They will keep for about 2 months in the refrigerator.