01 - Place the sliced celery and shallot into a clean pint jar or heatproof container, packing them tightly.
02 - In a small saucepan, combine the vinegar, water, sugar, salt, peppercorns, mustard seeds, and red pepper flakes. Bring to a simmer over medium heat, stirring constantly until sugar and salt dissolve completely.
03 - Once the brine reaches a simmer, carefully pour the hot liquid over the celery in the jar, ensuring the vegetables are completely submerged.
04 - Let the mixture cool to room temperature, then cover and refrigerate for at least 2 hours before serving to allow flavors to develop fully.
05 - Store in the refrigerator and use within 2 weeks for optimal quality and flavor.