1 pound rainbow chard (stalks sliced thinly and leaves cut crosswise into 1-inch ribbons)
1 small red (Bermuda) onion (very thinly sliced)
2 tablespoons olive oil
1/4 cup chicken stock or water
2 tablespoons toasted pine nuts
2 tablespoons dried red currants or raisins
  Kosher salt (to taste)
  Freshly ground black pepper (to taste)
  balsamic vinegar (to taste)