Cook potato in a large saucepan of salted water on high heat for 10-15 minutes, until tender. Drain and return to pan. Stir on medium heat for 1 minute, until dry. Mash, adding hot milk and butter. Season to taste.
Meanwhile, combine barbecue sauce, Worcestershire sauce and garlic in a shallow bowl. Add steak and turn to coat. Heat a lightly oiled char griller or frying pan on high. Reserving marinade, cook steaks for 2-3 minutes each side for rare, or until cooked to your liking.
Combine reserved marinade and stock in a small saucepan on high. Bring to boil and simmer for 2-3 minutes, until slightly reduced. Serve steaks with mashed potato and steamed green beans, and drizzle steaks with sauce.