Cut the beef into fine strips, season with salt & pepper. Heat up a wok with a couple of spoons of vegetable oil and seal the meat quickly, remove from heat. Next fry the vegetables together with the ginger, return the meat to the wok and moisten with enough beef stock. To make the crispy cups, cut the filo pastry into 15 cm square pieces (approx.) brush 3 pieces per cup with melted butter and stick on top of each other in a way that you obtain 12 corners. Press the filo sheet layers onto a muffin mould and bake until golden & crisp. Turn the pastry cases onto a serving plate; fill with the meat filling, top with the mascarpone cheese & crushed walnuts.