Place swiss chard stems and red onion in a mason jar or other vessel with a tight-fitting lid. Cut swiss chard stems to fit into mason jar as appropriate.
In a small bowl, whisk together the vinegar, sugar, and salt until fully dissolved. Stir in sriracha and celery seed, and then pour over the swiss chard and onions.
Cover jar with a tight-fitting lid and store in refrigerator for a couple days before using. Will last for a few weeks, kept refrigerator.
Use pickled swiss chard stems as toppings for tacos, sandwiches, as an accompaniment for rich meat dishes, or anywhere else a pickle seems appropriate 🙂 I recommend cutting the swiss chard stems into bite-sized pieces before serving.