Melt a knob of butter in a medium, nonstick frying pan, then cook the leek for about 5 mins until softened. Meanwhile, cut the potatoes in half, then into slices about ½cm thick. Beat the eggs, season, then stir in the cheese and sage.
Add a little extra butter to the pan if needed, tip in the potatoes, then the egg mixture. Turn the heat to low, then cook for
10 mins until nearly set. Place under a hot grill, then cook for a couple of mins more until the top is set and golden. Slice into wedges and serve with a green salad.