1 bunch chard stems (any color) chopped or kept long
2 teaspoons toasted brown mustard seeds
¼ cup white wine vinegar
¼ cup rice vinegar
3 tablespoons raw cane sugar
optional - sliced shallot
optional - a few pink peppercorns
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 stems and shallot in a jar, cover them with the brine and refrigerate overnight before eating them.