1. For the pastry, place the butter in the freezer for about 20 minutes until very hard. Sift the flour and a pinch of salt into a mixing bowl. Working quickly, hold the butter using a piece of baking parchment and coarsley grate it into the flour. Stir the butter and flour together, sprinkle with about 6 tbsp very cold water and mix to make a dough, adding a little more water if needed. Wrap in clingfilm and chill for about 30 minutes.
2. To make the filling, heat the olive oil in a large saucepan and cook the onion for about 5-8 minutes until really softened. Add the garlic and cook for a further minute. Stir in the spices, combine well then tip in the sweet potato.
3. Add the tomato purée and cook for 1 minute before adding the chopped tomatoes and about 150ml water. Bring to a gentle simmer and cook for about 15 minutes, until the sauce is thickened and the sweet potato is softened but still holding its shape. Set aside to cool.
4. When ready to make the pasties, roll out half of the pastry (it's easier to handle if you do it half at a time). Using a saucer, cut out a disc measuring 18cm in diameter. Repeat with the remaining pastry until you have 6 discs.
5. Stir the coriander into the sweet potato mixture. Spoon a heaped tablespoon of the mixture onto one side of each pastry disc and top with a spoonful of the ricotta. Brush the edges of the pastry with a little beaten egg then fold over to make a half moon shape. Seal the edges either with a fork or by pinching the pastry with your finger and thumb. Place on a baking tray lined with baking parchment.
6. Brush the top of each pasty with a little more beaten egg and sprinkle with nigella seeds. Bake for 20 minutes until golden and crispy. Delicious served with a simple mixed leaf salad and French dressing.
Recipe courtesy of Waitrose