Remove the heads from the herrings and scrape the skin from head to tail, to remove the scales.
Rinse well and dry with absorbent kitchen paper.
Cut along the underside of each fish, remove the roe and reserve for serving separately.
Scrape away and discard the gut.
Open out each fish and place it skin side uppermost on a board.
Press along the back of the fish to loosen the back bone.
Turn it over and gently ease away the backbone, starting at the head end.
Cut off the fins.
Mix the oatmeal and salt on a large plate and dip in each fish to coat thoroughly on both sides.
Melt the butter and oil in a frying pan and fry the herring for 5-8 minutes, turning the fish once half way through the cooking time.
Drain the fish on absorbent kitchen paper and serve immediately on a warmed serving dish with a garnish of lemon wedges and sprigs of parsley.