Chop your chard stems and sprinkle them with salt. Set aside.
Toast the brown mustard seeds in a small saucepan for a minute or so (don't let them burn). Add the vinegars and sugar. Bring to a boil, then reduce to a simmer. Stir until the sugar is dissolved. Remove pan from heat and let the brine cool.
Place the chopped stems and sliced shallot in a jar, cover them with the brine and refrigerate overnight before eating them.
I like to top mine on salads or in rice (or quinoa) bowls.
Recipe by Love and Lemons at https://www.loveandlemons.com/pickled-chard-stems/