Melt half the butter in a frying pan and cook the mushrooms until softened. Add the liver and cook for 5 minutes. Keep warm. Peel the potatoes. Heat the remaining butter in a frying pan and cook the potatoes until golden. Mix together the radicchio and Boston lettuce and divide it among 6 salad plates. Thinly slice the duck breasts and cook in a lightly oiled frying pan over high heat for 2 minutes. Spoon the mushrooms and liver mixture, the potatoes and duck over the salad In a small bowl whisk together the mustard, vinegar, oil, crÅme fraöche, salt and pepper. Spoon sauce over the salad and serve immediately.
