Cut the Swiss chard into large pieces, plunge it into boiling water and cook it until the stems are just tender.
Drain, shock under cold water and drain again. Squeeze out as much water as possible. Put the chard on a board and chop it up with a large knife.
Melt the butter or olive oil in a deep pot. Add the scallions and cook them over moderate heat until just tender. Add the chopped chard, toss well, season to taste and serve with a drizzle of cider vinegar.