Cut the veal kidneys into pieces. For lamb kidneys, make a V-shaped cut to remove the core from the center and cut each kidney into three or possibly four pieces. In a large frying pan, heat the oil with the butter over high heat and fry the kidneys heartily for three to four minutes, turning frequently to the other side. Transfer to a plate with a slotted spoon.
Add the quartered mushrooms to the frying pan and stir-fry until golden brown, about three min. Add chicken stock and brandy if desired and do two min. Stir in the crem fraiche and again simmer on low for three min until the sauce thickens a bit. Add the mustard and season well with salt and freshly ground pepper. Then add the kidneys and heat in the sauce for about a minute. Before serving, sprinkle chive rolls on top.
Tip: As an alternative to the fresh chives, you can also use the freeze-dried ones.