01 - Place the sliced celery and shallot (if using) into a clean pint jar or heatproof container.
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 to dissolve the sugar and salt.
03 - Once the brine reaches a simmer and the sugar and salt have dissolved, pour the hot brine over the celery in the jar, ensuring the vegetables are fully submerged.
04 - Let the mixture cool to room temperature, then cover and refrigerate for at least 2 hours before serving for best flavor.
05 - Store in the refrigerator and use within 2 weeks.