To prepare each mackerel, use a sharp knife to cut off the head just behind the gills, then cut along the belly so that the fish can be opened out flat.

Place the fish on a board, skin-side up, and, with the heel of your hand, press along the backbone to loosen it.

Turn the fish the right way up and pull the bone away from the flesh. Remove the tail and cut each fish in half lengthways. Wash and pat dry with kitchen paper.

Score the skin three or four times, then season the fish. To make the mustard and lemon butter, mix together the melted butter, mustard, lemon rind and juice and parsley. Season with salt and pepper. Place the mackerel on a grill rack. Brush a little of the butter over the mackerel and grill for 5 minutes each side, basting occasionally until cooked through.

Arrange the spinach leaves in the centre of four large plates. Place the mackerel on top. Heat the remaining butter in a small saucepan until sizzling and pour over the mackerel. Serve immediately.