Cook the potatoes in boiling water for about 10 minutes or until cooked but still firm.
Meanwhile, melt the butter with the oil in a large frying pan, add the leeks, cover and cook, stirring frequently, for 8–10 minutes or until soft. Stir in the peas.
Drain the potatoes, cut them into quarters and add to the frying pan. Continue cooking for 2–3 minutes.
Whisk the eggs with the milk and chives, season well and pour into the frying pan. Move around with a spatula so that the vegetables are well coated and the egg begins to cook. Crumble the cheese on top and leave over a medium heat for 2–3 minutes until the egg becomes firm.
Place under a preheated hot grill for 3–4 minutes until the omelette is completely set and the top is golden brown. Serve in thick slices with a prepared salad and ready-made dressing.