1. Sprinkle fillets of perch with salt and lemon juice and allow to marinate for 30 minutes.
2. Place a layer of duxelles of mushrooms in a shallow dish, lay the fillets on top, cover with white wine and fish stock and poach in the oven at 180°C for 10 minutes.
3. Remove the fillets and retain. Reduce the strained cooking liquor, add soured cream and seasoning.
4. Garnish with sliced boiled potatoes, coat with the sauce and serve.