Cook the cavolo nero, and keep warm.
Season the liver on both sides with salt and pepper. Brush a large frying pan with olive oil and, when very hot, fry the liver for 1 minute on each side. Add the balsamic vinegar and turn the liver so that it absorbs the vinegar which will reduce almost immediately. Add the crème fraîche and let it melt into the vinegar. Remove and serve on top of th