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, stir in the cheese and sage.
Add a little extra butter to the pan if needed and tip in the potatoes, followed by the egg mixture. Turn the heat to low, cook for 10 mins until nearly set. Place under a hot grilland cook for a couple of mins until the top is set and golden.
Slice into wedges and serve with a green salad