Preheat the oven to 190ºC / 375ºF / Gas Mark 5. Mix the oil, garlic and creme fraiche together
in a large bowl, add the sweet potato slices and toss until well coated.
Melt the butter in a non-stick frying pan and cook the leeks until soft. Add to the sweet
potatoes and mix until evenly distributed. Season to taste with pepper.
Lightly grease a grating dish. Layer in the sweet potato mixture and top with the cheese and
Cover with foil and bake in the preheated oven for 1 hour, removing the foil for the last 5
minutes of the cooking time.