preheat oven to 190.C/375 F/Gas 5. Dry the sweet potatoes with kitchen paper and rub them all over with 1 of the 2 tbsp of olive oil. Place them on a baking sheet ,sprinkle with salt. Bake for about 1 hour until tender.

Meanwhile, heat the rest of the oil in a frying pan and add the sliced leeks.Cook for 3-4 minutes or until softened and just beginning to turn golden.

Cut the potatoes in half lengthways and place them cut side upon the baking sheet.Top with the cooked leeks and season.

Lay the cheese slices on top and grill(broil) under a hot grill for 2-minutes,until the cheese is bubbling. Serve immediately.