Bring potatoes to a boil, then reduce the heat and simmer for 10 to 12 minutes. Meanwhile, heat oil in a non-stick wok or large frying pan over medium-high heat. Add mushrooms, peas, and pepper strips, and stir-fry for 1 minute. Mix half of the broth 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 steaks on both sides with coarsely ground black pepper; set aside. Heat a grill pan. Meanwhile, drain cooked potatoes and add to vegetables. Stir gently; cover and keep warm.
Add butter to the grill pan and turn heat to high. When the butter starts to foam, add steaks. When cooked to your preferred doneness, put the steaks onto warmed dinner plates. Keep warm.
Add shallot and garlic to the cooking juices in the pan and cook, stirring, over 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 broth and boil 1 minute. Correct the seasoning. Spoon sauce over the steaks and serve immediately with the vegetables.
Per serving: 343 calories, 32 g protein, 9 g total fat, 3 g saturated fat, 83 mg cholesterol, 32 g carbohydrates, 4 g fibre.