Heat 2 tsps of the olive oil in a heavy pan and add the onion. Cook, stirring over a medium heat for 5 mins until soft. Add the garlic and the curry powder and cook, stirring, for a few mins more.
Add the sweet potato and apple and stir well to combine. Cook, stirring frequently, until the sweet potato starts to brown at the edges, about 15 mins. Keep adjusting the heat to prevent the onion from burning.
Pour in 1 ltr of the stock and stir, scraping up the caramelised bits from the bottom of the pan. Gently simmer, uncovered for about 20 mins, or until the sweet potato is tender.
Carefully transfer to a blender and blitz until smooth. Pour into a clean pan and add more of the remaining stock if the soup is too thick for your liking. Add the coconut milk and lime juice and season with salt and pepper. Serve immediately, sprinkled with the croutons
To make the croutons, heat the remaining olive oil in a frying pan and add the pumpernickel. Cook over a medium heat, shaking the pan, until the bread starts to crisp up. Remove to paper towels to soak up excess oil