Heat the butter, sauté the onion cubes and the finely chopped leek and fill up with the fish stock. Let simmer for 8 minutes, should boil down a little thick. Add the cream and let it boil down until creamy. Stir in the mustard, puree with the cutting stick and insert the fish fillets. Put the heat source down and let the fish steep for 8-10 minutes.
Then add the prawns and let stand for another 3-4 minutes. Test that the fish is done. Season the sauce with the tarragon puree, salt and pepper.
To the tarragon puree:
I have a lot of tarragon this year and have pureed the leaves with a lot of salt and olive oil. Of course, you can also just use fresh tarragon leaves.
With this recipe you can play a bit, more mustard, more tarragon, less cream, it`s all a matter of taste. It is important to let it boil down.