Heat the oil in a wide pan. Sauté the garlic, lemongrass and curry paste. Add the potatoes and leek, cook briefly. Pour in the water, bring to the boil, season with salt. Reduce the heat, cover and simmer for approx. 1 hr. Remove the lemongrass, add the crème fraîche, blend the soup.
Heat the oil in a non-stick frying pan, add the potatoes, season, stir fry for approx. 10 mins. Plate up the soup, top with the crème fraîche, sprinkle the potato cubes on top.