Skin and core the kidneys with a pair of scissors.
Mix flour, cayenne pepper and mustard powder together in a bowl.
Toss the kidneys in the flour and shake off the excess.
Preheat a frying pan; add 2 teaspoons of the oil and the butter. Fry the kidneys for 3-4 minutes until golden. Remove from the pan and set aside.
Add the remaining oil to the pan with the shallots, garlic and mushrooms and sauté over a medium heat for 2-3 minutes. Add a few drops of Worcestershire sauce to the pan, pour in the stock and reduce by half. Add the crème fraîche and kidneys to reheat for 1 minute and season to taste.
Finally, add the chopped parsley an extra sprinkling of cayenne pepper and serve on hot buttered toast.