1 tablespoon dry mustard (such as Colman's) dissolved in a little wate
2 tablespoons whole-grain mustard
Salt and ground black pepper
In a medium saucepan melt the butter over low heat, add the mushrooms and shallots, and cook gently for 2 minutes. Stir in the curry powder and add the brandy and the wine. Bring the mixture 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 mustard and cook until the sauce is thick enough to coat the back of a spoon.
Strain the sauce through a fine-mesh sieve, then stir in the whole-grain mustard. Season to taste with salt and pepper.