Add each half of the celery seed, peppercorns, mustard seeds, and garlic to each jar. (If just making one jar, this can all go together.)
Pack chard stems tightly into jars.
Bring water, vinegars, sugar (or maple syrup), salt, and sriracha to a boil, in a small saucepan until sugar and salt is dissolved. Then pour over chard stems.
Let sit until cool, then put lids on and refrigerate. Wait two days before eating for flavors fully develop. Will last 6 weeks in refrigerator. To eat, chop or dice them (important) and add to tacos, salads, as a relish for a sandwich, in omelets.
