Wash the fish fillets. Rub a flat saucepan with the butter and sprinkle the finely chopped onion cubes over it. Now place the fish fillets side by side. Season with salt, pepper, tarragon and parsley and pour over white wine and lemon juice. Place a parchment paper over it (a saucepan lid works too) and let the fish fillets simmer for about 5 minutes. Arrange the cooked fish fillets.
In the meantime, mix the cream with the cornstarch and add to the fish stock. Bring it to the boil briefly and season the fish sauce to taste with aromatic or fondor. Serve with the fish fillets.
Serve as a side dish with rice or boiled potatoes and a green salad of your choice.