Heat the oil in a large, heavy-based frying pan and fry the onion for 1-2 minutes or until soft. Add the garlic, chilli and paprika and cook for a further 2 minutes, or until aromatic, then add the sweet potato to the pan and toss to cover with the spices.
Pour in the stock, bring to the boil, then reduce the heat and simmer for 15 minutes, or until the vegetables are just tender. Remove from heat and allow to cool slightly, then transfer to a blender or food processor and blend (in batches, if necessary) until smooth. Add extra water if needed to reach the desired consistency, but do not overblend or the mixture will become gluey. Season to taste with salt and cracked black pepper.
This soup can be served immediately, ladled into bowls and garnishe'd with a few dried chilli flakes. Alternatively, leave it to set on the stove, then gently re-heat when needed: this will allow the flavours time to combine (and will really bring out the heat!).