Preheat the oven to 200C/400F/Gas 6. Lightly grease a small ovenproof dish.
Place the sweet potato slices into a saucepan with the stock and a little extra water to cover the sweet potato, if necessary. Bring to the boil, reduce the heat and simmer for 7-8 minutes, or until the sweet potato is just tender, then drain.
Arrange the sweet potato pieces overlapping in the buttered dish. Crumble over the goats' cheese, scatter over the rosemary leaves and season with salt and freshly ground black pepper.
Transfer to the oven and bake for 6-8 minutes, or until the cheese is melted and golden-brown. Serve warm.