To make the leek and mustard mash, place the potatoes in a saucepan and pour over boiling water to cover by 5cm. Bring back to the boil, then reduce the heat and cook for 15–20 minutes or until the potatoes are very tender.
Meanwhile, spread out the crushed peppercorns on a plate and press the steaks into them until they are coated with peppercorns on all sides. Set aside.
Heat the oil for the mash in a nonstick frying pan. Add the leeks and cook, stirring constantly, for 3–5 minutes or until tender. Transfer to a plate lined with a double thickness of paper towel to drain. Heat the milk in a saucepan until hot.
When the potatoes are tender, drain them, shaking the colander or sieve to remove any excess water, and return them to the pan. Pour the hot milk over the potatoes then mash them until they are completely smooth. Add the leeks, mustard and margarine and season with pepper. Beat well to mix, then cover and keep warm.
Heat a ridged cast-iron grill pan over a high heat until hot. Brush the pan with the olive oil, then reduce the heat to medium-high. Place the steaks in the pan and cook for 3 minutes on each side for rare, 3 1/2 minutes on each side for medium-rare, 4 minutes on each side for medium, or 5 minutes on each side for well done.
Spoon a mound of mash on each warmed plate and place a steak next to it. Drizzle any pan juices over the steaks and sprinkle with parsley or chives. Serve immediately.