01 - Pack the sliced celery and shallot into a clean pint jar or heatproof container, pressing gently to maximize space.
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 - Carefully pour the hot brine over the celery, ensuring all vegetables remain fully submerged in the liquid.
04 - Allow the mixture to cool completely to room temperature, then cover tightly and refrigerate for minimum 2 hours to develop optimal flavor.
05 - Keep refrigerated and consume within 2 weeks for best quality and texture.