Heat the oil in a frying pan over medium heat and fry the grated carrot and peas for a couple of minutes, stirring often.
Add the mashed potato and cook for a couple of minutes.
Add the ground coriander, ground cumin, ground turmeric, garam masala, onion powder, chilli powder, if using, and salt. Stir well. Take off the heat and leave to cool for 20 minutes.
Using a hand blender, blend until smooth (although a few lumps of pea can remain).It will be very thick and sticky, but that it ok. Preheat the oven to 190˚C fan/ 210˚C/ 374˚FLine a baking sheet with baking paper.
Spread the spicy potato mixture evenly over the puff pastry sheet – you may need to use your fingers as it can be quite thick.
Taking one side, carefully and tightly roll.
If using the nigella seeds, sprinkle them in an even layer over your chopping board, then gently roll the puff pastry roll in them to get an even coating all the way around.
Optional step: place in the fridge for 15 minutes this will make the pinwheels easier to cut.
Using a sharp knife, cut into 1 cm / .4" thick slices and lay them cut side down on the tray.
Place in the oven and cook for 18-20 minutes until lightly golden brown.
Cool for 10 minutes before serving.
Best served on the day they are made but can keep, covered in the fridge for up to 3 days.