Ask your butcher to remove the grease from and to peel the kidneys.
Remove the extremity of the mushrooms and wash them in water with a little lemon juice.
Fry the kidneys, over a high heat, in 30g of butter, but allow them to remain rare.
Remove and keep warm.
Throw away the fat from the pan; add the remaining butter, the quartered mushrooms, cover and leave to cook for 5 or 6 minutes.
Remove the mushrooms.
Deglaze the cooking juice of the mushrooms with the brandy.
In a bowl, mix 2 teaspoonfuls of each mustard and the fresh cream.
Pour into the pan, allow to boil for a few seconds and replace the kidneys and the mushrooms.
Allow to heat completely, over a low heat.