01 - Place the sliced celery and shallot into a clean pint jar or heatproof container, packing them tightly but leaving room for the brine.
02 - Combine vinegar, water, sugar, salt, peppercorns, mustard seeds, and red pepper flakes in a small saucepan. Bring to a simmer over medium heat, stirring constantly until sugar and salt completely dissolve.
03 - Once the brine reaches a gentle simmer, carefully pour the hot liquid over the celery in the jar. Ensure the vegetables are completely submerged in the brine.
04 - Allow the mixture to cool completely to room temperature, then cover with a tight-fitting lid. Refrigerate for at least 2 hours before serving to develop optimal flavor.
05 - Keep refrigerated and consume within 2 weeks for best quality and texture.