Lightly season the fish fillets or otherwise the cod fillets with salt and season with pepper. Butter the bottom of a steamer basket. Put the fish in it. Pour the fish stock into the saucepan and place the steamer on top. Steam the fish fillets or otherwise the cod fillets for 3-4 minutes with the lid closed.
In the meantime, finely chop the Ascalon onion. Sauté in olive oil until light yellow. Add Noilly Sherry or alternatively Prat and white wine.
Keep the cod legs or alternatively the fish fillets warm with the lid closed. Measure out 50 ml of the gravy and add to the sauce liquid. Boil it up to half the quantity on a high fire.
Add vinegar and mustard. Let the sauce boil on the stove until it starts to thicken. At the end stir in the heavy cream. Season the sauce with salt and pepper. Mix the fish thoroughly with the sauce and serve.
Tip: Feel free to use better olives – it pays off!