Preheat the oven to 180°C.
1 Filling Arrange the sweet potato slices on a baking sheet, sprinkle a little oil over, season with salt and pepper and roast until soft.
2 Heat a little oil in a pan and fry the onions until soft. Sprinkle the sugar over and stir-fry until caramelised. Sprinkle the vinegar over and season with salt and pepper.