Heat the oil and butter in an ovenproof frypan, about 20cm across, until butter is melted. Add the leek and a decent pinch of salt. Cover and partially cook, stirring occasionally, about 15 minutes or until leek is tender.
Meanwhile, preheat grill. Whisk together the eggs and sour cream and season.
Spread the leek evenly over the base of the pan. Sprinkle with the dill then pour the egg mixture over. Cook over medium low heat for 5-8 minutes or until you judge it is half cooked.
Place the frying pan under the grill for 3-4 minutes or until just set. Serve warm in wedges.