Prep all your ingredients. One of the most important steps to cooking is to have everything ready to go before you start. This tip can save you from lots of potential mistakes.
Heat a saucepan over medium heat and when hot, add the butter and let it melt.
Add the shallots and sweat (cook but do not brown) for a minute or two, making sure you are staring so they do not brown.
Add the mushrooms and saute for a few minutes until they release moisture, but don’t let them brown.
Stir in the curry powder and add the cognac or Armagnac and wine. Bring to a boil, and reduce the liquid by one-third.
Pour in the fish stock, simmer for 5 minutes, then add the cream and the reconstituted mustard and cook until the sauce is thick enough to coat the back of a spoon.
Season to taste with salt and pepper and pass the sauce through a wire-mesh conical strainer. Stir in the whole-grain mustard.
The sauce is now ready to serve.