Melt 15 g (½ oz) of butter and cook the vegetables, bacon and tarragon without colouring until they soften. Add the white wine and boil to reduce until almost dry. Add the stock and boil to reduce by half. Add the cream and cook for 8–10 minutes. Strain the sauce and whisk in the remaining butter. Add the mustard a little at a time until you have the right strength.
Meanwhile, brush the herrings with a little oil or butter and cook under a hot grill for 5–6 minutes.
Sit the potato cakes in the centre of the plates, spoon the sauce around and lay the herring fillets on top.