For the sauce, mix the sour cream and low-fat yoghurt with the finely chopped cucumber and finely chopped onion. Stir the herbs into the sauce. Season well with lemon juice, Worcestershire sauce, salt and pepper.
Drizzle the fish fillets with Worcester sauce, season with salt and pepper. Let the fish marinate for 10 minutes. Dust the fish with flour.
Whisk the eggs, parmesan and parsley into a breading. Turn the fish in the breading and fry in the butter until golden brown on both sides.
Serve with boiled boiled potatoes and the herb sauce.