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Heat the oven to 200C/180C fan/gas 6. Pierce the sweet potatoes all over with a fork, then cook in the microwave on high for 8-10 mins until softened. Or, bake in the oven for 50 mins-1 hr, turning halfway through. Leave to cool a little, then cut in half and scoop the flesh into a bowl. Leave to cool completely.
Meanwhile, heat the oil in a frying pan over a medium heat and cook the red onions and a pinch of salt for 10-12 mins, stirring occasionally until softened and caramelised. Add the garlic, harissa and tomato purée and fry for 2 mins more. Tip the mix into the bowl with the sweet potato along with the coriander, then mash together. The filling should be mostly smooth, but not a purée. Season and leave to cool completely.
Unroll the pastry sheet and cut in half lengthways into two long rectangles. Spoon half the filling down the middle of one of the rectangles and form it into a long sausage shape. Brush the edges of the pastry with a little of the milk, then fold the empty half of the pastry over the filling and press the edges to seal. Crimp using a fork to ensure the filling is enclosed, then repeat with the remaining filling and remaining rectangle of pastry.
Cut the pastry along the length into 7cm-long rolls, then arrange, spaced apart, on a large baking tray lined with baking parchment. The rolls can now be frozen on the tray. Once solid, put in a freezerproof container and freeze for up to three months. Brush with more milk, then sprinkle with the nigella seeds. Cut small slashes into the top of each roll. Bake for 25-30 mins (or 40 mins from frozen), turning halfway through, until golden.
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