Cut the steak almost in half lengthwise to form a pocket for the filling. Heat the butter in a frying pan and fry the onion for 3 minutes until softened. Add the mushrooms and cook over moderate heat for 5 minutes. Remove from the heat and stir in all the remaining ingredients except the oil. Sandwich the stuffing into the cavity in the steak. Sew the 2 sides together with string or secure with toothpicks. Brush the steak on both sides with oil. Grill under the griller or over charcoal 5 minutes on each side.