1. Melt the butter in an 18 cm (7 inch) heavy-based omelette or frying-pan, add the leek and fry for about 5 minutes, until soft but not browned.
2. Beat the eggs lightly with a fork, add the leek and corn and season well with salt and pepper.
3. Heat the oil in the same pan, add the egg mixture and cook over a very low heat for about 12 minutes, until the frittata is set and the underside is golden brown.
4. Either slide the frittata onto a plate, cover with a second plate, turn over, slide the frittata back into the pan and cook for 4-5 minutes, or place the pan under a broiler for 2-3 minutes, to brown the top.
5. Serve with a crisp salad and whole wheat bread.