I have this regularly for supper, and I tend to make life easier for myself by always having a stash of baked sweet potatoes in the fridge. If you don't, I advise baking the potatoes the night before, so that when you come to make this soup the next or another night, you are about 3 minutes away from a soothing, comforting, gutsily-flavoured big bowlful. But even if you do roast the sweet potatoes to order, as it were, you need do nothing while they're in the oven, and not much after. For US…