Strip the green leaves from the chard stalks so that you are left with only the white ribs. Pull away any strings and strip off the thin skin covering them, then cut them into strips 4cm (1½in) by 2 cm (¾in). Wash and drain them.
Mix two egg yolks in a bowl with 1 tablespoon cream.
Heat the butter in a casserole, add the chard, cover the pan and cook