Wash your fish filets and marinate them in lemon juice for around 10 minutes. Melt the butter in a sufficiently big pan for your filets. Stir the flour in the melted butter and fry for 1 minute on low flame. Stir the mustard and the cream into the flour mixture. Gradually add the vegetable stock, mixing thoroughly to avoid clumps. If you don’t have vegetable stock you can use water with a dash of white wine. Add salt and pepper according to taste and bring the sauce to a slow boil. Once the sauce has thickened – which should happen after 2 or 3 minutes of boiling – add the fish filets which should be covered by sauce. Simmer for around 7 minutes, garnish with chopped parsley and serve with boiled potatoes.