Begin by cleaning the rainbow chard stems thoroughly and trimming them to fit your mason jars. Aim for pieces about 4-5 inches long.

In each jar, distribute the celery seeds, black peppercorns, yellow mustard seeds, and garlic evenly.

Tightly pack the cut chard stems into the mason jars, pressing them down gently.

In a medium saucepan, combine water, rice wine vinegar, distilled white vinegar, sugar (or maple syrup), salt, and sriracha. Bring to a boil, stirring until sugar dissolves.

Carefully pour the hot brine over the packed chard stems in the jars, ensuring they are completely submerged.

Allow the jars to cool to room temperature before sealing them with lids.

Refrigerate the jars for at least 2 days to allow flavors to develop.