Method
Preheat oven to 200ºC/ Gas 6
Boil the potatoes in salted water. Drain and mash with butter, milk and cheese, and season to taste.
To make the sauce, melt the butter and sauté the shallot for 5–6 minutes until soft. Add the white wine vinegar and boil for 2–3 minutes until the liquid has evaporated. Stir in the flour to make a roux. Gradually stir in the fish stock and then cook over a medium heat, stirring continuously. Bring to the boil, stirring until thickened. Remove from the heat and stir in the cream and chopped chives. Season well.
Remove the skin from and cut into 2 cm chunks. Divide all the fish and peas between 4 ramekins and pour over the sauce. Cover with mashed potato. Brush the potato topping with a little beaten egg.
Bake in the oven for 25 minutes.