Add apple cider vinegar, sugar, juniper, black peppercorns and bay to a medium, non-reactive (not aluminum) pan set over medium-high heat, stirring until the sugar dissolves. Bring mixture to a boil, then reduce heat to medium-low.

Peel, quarter and core each quince, then cut each quarter into 3 slices. Carefully drop the slices into the simmering vinegar, and cook until tender (20 - 30 minutes). An inserted fork should meet no resistance.

Using a slotted spoon, transfer quince to a large, clean jar (or several small ones), then pour the pickled liquid over top. The fruit should be submerged in the liquid. Let the fruit cool to room temperature, seal the jar, and store in the refrigerator (for up to a month).