I love Nigella's recipes, but don't know when you would expect to serve the Sweet Potatoes With Marshmallows? As a dessert?
The recipe for Sweet Potatoes With Marshmallows (from FEAST) is Nigella's version of a dish that appears at Thanksgiving, where is is often known as sweet potato casserole. So it is very much a side dish rather than a dessert. It would also be suitable for serving with a ham, as a salty and sweet combination.
It is thought that the topping of marshmallows originated around 100 years ago, as a recipe printed on the back of marshmallow boxes. Sweet and savoury is not unusual at Thanksgiving and candied yams are another popular side dish, where sliced yams (a type of sweet potato) are baked with butter and sugar until the yams are soft and coated with a thick syrup. Sometimes this dish is also finished with marshmallows on top.