Swedish-Style Marinated Herrings

This must be made with the freshest herrings, bright-eyed and firm as cucumbers. An obliging fishmonger will fillet them for you. Then all you need do is cut them diagonally into neat strips and marinate them in this sweet and spicy Swedish-inspired ‘cure’ for 48 hours or so. I find standard roll-mop marinades harshly vinegary; the Scandinavian additions of sugar, mustard and spices transform this into an elegant and unusual starter, served with half a hard-boiled egg, a slice of dark rye b