Cut 2 thin slices out of the middle of the lemon and reserve them for the garnish.
Sprinkle the juice from the remaining lemon over the herrings.
Sprinkle some salt and black pepper over them.
Peel and slice the potatoes and boil them until tender.
Preheat the grill and cook the fish until it is tender.
This will take about 8 minutes per side.
Cut the cooked herrings into 1 inch (3 cm) pieces.
Melt the butter in a saucepan and whisk in the flour.
Keep cooking and whisking for 2 minutes, then take the mixture off the heat and stir in the hot milk.
Add the mustard, wine and a little salt to taste.
Put the sauce back in the pan and bring it to the boil.
Turn the heat down and let it simmer for a minute or until the mustard sauce is thick.
Pour the sauce on to a serving dish and arrange the potato slices on top.
Arrange the herring slices over the potatoes.
Peel the onion and slice it thinly, separating the slices into rings.
Garnish the potato and herring starter with the onion rings, bread cubes, lemon slices, gherkin and the sprigs of fresh parsley.
Serve immediately.
