Rinse fish, pat dry. Finely dice the smoked salmon. Mix with butter, lemon zest, dill, breadcrumbs, salt and pepper. Spread the mixture over the fish. Bake in a preheated oven (250 degrees electric stove, gas mark 5) for about 10 minutes.
Peel and dice shallot. Braise in hot fat. Add the stock and wine. Reduce to half over high heat. Add the sour cream and horseradish. Season to taste with salt and pepper.