Put the potatoes in a saucepan and cover with boiling water. Bring back to the boil, then reduce the heat and simmer for 10–12 minutes.

Meanwhile, heat the oil in a wok or large frying pan (preferably non-stick), add the mushrooms, peas and pepper strips, and stir-fry for 1 minute. Mix 120 ml (4 fl oz) of the stock with the Worcestershire sauce, mustard and sugar, and stir into the vegetables. Reduce the heat and simmer gently for 3 minutes or until the vegetables are just tender, stirring frequently.

Season the steaks on both sides with coarsely ground black 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 a very low heat until ready to serve.

Add the shallot and garlic to the cooking juices in the pan and cook, stirring, over a low heat for 1 minute. Pour in 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 a minute. Check the seasoning. Spoon the sauce over the steaks and serve immediately, with the vegetables.

Put the butter into the hot grill pan and turn up the heat to high. As soon as the butter sizzles and starts to foam, add the steaks. The cooking time depends on the thickness of the meat and whether you like your steaks rare, medium or well done. For steaks 1 cm (½ in) thick, allow 1 minute on each side for rare (it will feel springy when pressed), 1½–2 minutes on each side for medium (it will feel slightly resistant when pressed), and 2½–3 minutes on each side for well done (the meat will feel firm when pressed). Lift the steaks onto warmed dinner plates, and keep warm while making the sauce.