Begin by peeling the onion and slicing it into very thin rings. Now slice the rings in half and separate them to give you moon-shaped slivers. Melt 2oz (50g) of the butter in a large frying pan and gently soften the onion in it for 10 minutes.
Meanwhile, wipe the mushrooms, slice the stalks and caps thinly, then add them to the onion, stirring them around, and cook for a further 5 minutes. The kidneys should be first skinned and the cores snipped out then cut into the thinnest slices possible. Turn the heat up under the pan, add the kidney slices and brown them quickly, constantly turning the pieces over.
Now turn the heat right down and stir in the soured cream, season with salt, pepper and nutmeg, and let everything simmer very gently for 5 minutes.
Just before serving, stir in the remaining butter, then serve immediately with plain boiled rice.