Preheat the oven to 190 degrees fan, 200 degrees non fan
Heat the oil in a frying pan and add the mustard seeds. As soon as they start to pop add the onion and fry over a medium heat until starting to turn golden.
Stir in the spices and salt
Add the mashed potato and peas and mix well. Let cool for a few minutes
Roll out the puff pastry and lay it horizontally to yourself
Spread the mashed potato mixture over the whole of the sheet, leaving a 1cm gap on the edge closest to you
Roll the pastry towards yourself to make a large swirl filled log
Cut into twelve even slices and place well apart on a lined baking sheet
Bake for 18-20 minutes until golden and risen
Serve hot or leave to cool on a wire rack