Melt the butter together with the oil and gently fry the garlic, leek and sweet potato for several minutes. Add the spices and cook for a further minute.
Add the sea salt and pepper, vegetable stock, tinned tomatoes and tomato puree and cook until the sweet potato has softened (about 30 minutes).
Remove from the heat and blend until smooth. Add more water if required. Pour back into the pan and heat through again. Serve immediately with some nice, fresh bread. Enjoy!