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Heat oven to 180C/160C fan/ gas 4. To make the pastry, tip all the ingredients, plus 2 tbsp salt, into a food processor and pulse to mix. Add up to 50ml water, splashing it in gradually and continuing to pulse, until the dough sticks when you pinch it.
Roll out the pastry on a floured surface to the thickness of a £1 coin – it should be quite stiff. Cut out 20 circles using a 7cm biscuit cutter (re-rolling the trimmings if needed) and neatly encase each potato, stretching the pastry a little if you need to so that the potatoes are completely encased. Arrange the potatoes on a baking tray and bake for 40 mins until the pastry is solid and the potatoes are soft when prodded with a skewer – the potatoes will stay hot for 15 mins.
While the potatoes are baking, make the mayo. Tip the egg yolk and mustard into a bowl with some seasoning and blitz together using an electric whisk, while very gradually pouring in the oil until you have a thick mayo. Add the seaweed and lemon juice.
Serve the potatoes in a pile with a bowl of mayo on the side. To eat, simply crack open the pebble and dip the hot potato into the mayo – dip and eat the pastry too, if it’s to your taste.