In a bowl cream together the cheese and butter until the mixture is smooth. In a saucepan boil the wine with the peppercorns until it is reduced about 1 tablespoon, add the cream, and boil the liquid until it is reduced by half. Reduce the heat to moderately low, whisk the cheese mixture, a little at a time, into the cream mixture, and whisk in the parsley. Remove the pan from the heat and keep the sauce warm.

Preheat a grill pan. Pat steak dry and season with salt and pepper. Grill steak 4 to 5 minutes on each side for medium rare. Let the steak stand on a cutting board for 10 minutes. Slice thin and serve with the Stilton sauce and watercress.