Heat the stock in a small pan and add the salmon and white fish. Simmer gently for 5 mins, then turn off the heat and leave to sit for 5 mins more. Remove the fish, keeping the stock, and flake into large chunks, removing bones as you go. Spread onto the base of 1 large or 2 smaller overnproof dishes with the shrimp, dill and green beans.
Pour the wine, reserved stock and cream into a pan and simmer to reduce a little. Mix the cornflour to a paste with 1 tbsp of the sauce and whisk back into the sauce, simmering until thickened. Pour over the fish and veg and leave to cool a little while you make the topping.
Boil the potatoes in salted water until tender, about 10-15 mins. Drain and steam-dry for a few mins, then return to the pan with the olive oil, chives and half the cheese and chives. Season, crush lightly with a fork or potato masher, then spoon over the fish. Sprinkle on the remaining cheese and chives.