Melt the butter in a heavy saucepan and stew the onion and celery together gently for 10–15 minutes until transparent. Add the yellow peppers and continue to stew gently for another 10 minutes.
Meanwhile, shave the sweetcorn off their cobs by standing them upright on a board and cutting down with a large sharp knife. Add the corn to the pan, together with the thyme and bay leaf. Add the