Cut the bread into six medium thick slices. Spread three of the slices with butter (if using) and top this with the wafer-thin roast beef then a handful of the watercress.
Mix together the horseradish sauce, créme fraîche and lemon juice and season with a little salt and plenty of ground black pepper. Spread this on the other three slices of bread and place over the roast beef and watercress, pressing down gently. Cut each sandwich into 4 triangles and serve.
Cook's tip: skip buttering the bread; and use a little extra creamed horseradish sauce without adding the crème fraîche and lemon juice.