Prepare Jars: Begin by washing and drying 2 pint jars. Pack the rhubarb pieces and onion slices snugly into each jar for optimal flavor infusion.

Make Brine: In a medium saucepan, combine the apple cider vinegar, water, granulated sugar, kosher salt, mustard seeds, black peppercorns, and red pepper flakes. Heat the mixture gently for about 4-5 minutes, stirring until the sugar and salt dissolve completely.

Combine: Carefully ladle the hot brine over the packed rhubarb and onions. Ensure that the vegetables are mostly submerged for even pickling. Tap the jars gently to release any air bubbles trapped inside.

Cool: Allow the jars to sit uncovered for about 30-45 minutes, letting them cool down and allowing the flavors to mingle harmoniously.

Refrigerate: Seal the jars tightly and store them in the refrigerator. For the best flavor, wait at least 24 hours before diving in.