Thoroughly heat a large saute or frying pan. Add a scant 1/2 oz of the butter and tilt so that the fat runs sizzling all over the pan base. Add the mushrooms (whole if small or sliced thickly if large) and saute over high heat for 5 minutes. Remove and keep hot. Add the rest of the butter to the pan, then the onions which should be thinly sliced and pushed into rings. Cook very gently indeed for 15-20 minutes until beautifully softened. Meanwhile stir the yoghurt into the soured cream and season very generously with salt and pepper; trim the chicken livers and pat them dry with kitchen paper. Lift the onions out of the pan with a slotted spoon, so all the buttery juices drip back into the pan, and keep them hot. Increase the heat under the pan and saute the chicken livers for about 3 minutes so they become crusted with brown on the outside, but remain deliciously pink and tender within. Return the mushrooms and onions to the pan and immediately pour on the seasoned soured cream. Let the sauce bubble up and cook for a few minutes until it is very hot, slightly reduced and thickened. Stir gently every now and again to encourage the sauce to cling creamily to the meat and vegetables. Check seasoning and serve without delay. Green beans and triangles of fried bread go well with this dish, and I like to accompany it with plenty of crusty hot French bread to mop up every luscious drop of sauce. More substantially, you could substitute rice or thin ribbons of pasta for the bread.