Place the potatoes in a pan and cover with boiling water. Bring back to the boil, reduce heat and simmer 10–12 minutes.
Meanwhile, heat the oil in a frying pan, add the mushrooms, peas and capsicum and stir-fry for 1 minute. Mix 1/2 cup stock with the Worcestershire sauce, mustard and sugar, and stir into the vegetables. Reduce heat and simmer gently for 3 minutes or until the vegetables are just tender, stirring frequently.
Season the steaks on both sides with pepper and set aside. Heat a ridged cast-iron grill pan. Meanwhile, drain the cooked potatoes and add to the vegetables. Stir gently, then cover and leave over very low heat until ready to serve.
Place the margarine in the hot pan and turn up the heat to high. When the margarine starts to foam, add the steaks. For steaks 1 cm thick, allow 1 minute on each side for rare, 1 1/2–2 minutes on each side for medium, and 2 1/2–3 minutes on each side for well done. Lift the steaks onto warmed dinner plates. Keep the steaks warm while making the sauce.
Add the shallot and garlic to the juices in the pan and cook, stirring, over low heat for 1 minute. Add the port and increase the heat so the sauce is bubbling. Cook for about 1 minute, stirring. Pour in the remaining stock and let it bubble for 1 minute. Spoon the sauce over the steaks and serve immediately, with the vegetables.