Preheat oven 180°C fan forced. Heat half the oil in saucepan over high heat. Add mushrooms and garlic, cook for 5 minutes or until mushrooms are tender. Combine the Worcestershire sauce, stock and mustard in a jug. Stir into the mushrooms, cook, stirring occasionally for 3 minutes.
Blend the cornflour with water until smooth. Stir the cornflour mixture into mushrooms. Bring the sauce to the boil, stirring constantly. Reduce heat to medium and cook, stirring occasionally, for 5 minutes or until sauce thickens slightly. Stir in the parsley.
Meanwhile, heat char-grill pan over high heat. Rub both sides steaks with remaining oil. Char-grill for 2-3 minutes each side until seared. Transfer to the oven and cook a further 5 minutes for medium or until cooked to your liking. Transfer to a warm plate, cover to keep warm while resting for 5 minutes.
Spoon mash potato onto plates, top with steak. Spoon over mushroom sauce, serve with steamed vegetables, if desired.