Cheese, potato and onion rolls

Method

1. Start with the pastry if you’re making it (see tips). Put the flour in a large bowl, whisk in the mustard powder and salt, then grate or dice in the cold butter. Rub the butter into the flour with your fingertips until it looks like breadcrumbs, then add just enough cold water (about 3 tbsp) to bring everything together into a smooth dough. Wrap well and chill (see Make Ahead).
2. For the filling, put the potatoes in a pan of salted water and bring to the boil. Cook for 3-4 minutes until almost cooked. Drain, then steam dry in the hot pan for 2 minutes.
3. Meanwhile, in a medium frying pan, fry the onions in the butter over a low-medium heat until soft and golden but not browned (it will take about 10 minutes). Set aside to cool slightly, then roughly chop.
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4. Mix the potatoes, onions, nutmeg and some salt and pepper in a large bowl until they come together to form a rough mash. Add the cheese, then mix again (see Make Ahead).
5. Heat the oven to 220°C/200°C fan/ gas 7. Roll out the pastry on a lightly
   floured surface to a 30cm x 40cm rectangle, about 5mm thick, then cut lengthways into 2 equal strips.
6. Spoon even lines of filling down the length of each piece of pastry, just to the left of the centre line. Brush the exposed pastry with beaten egg, then fold over to enclose the filling with the join on the underside.
7. Cut into equal lengths to make 18-20 rolls, then put on the prepared baking tray. Brush all over with beaten egg, sprinkle with nutmeg, then chill for 30 minutes. Bake for 20-25 minutes until golden. Leave to cool slightly before serving (see Make Ahead).