Heat oven to 220C/fan 200C/gas 7 and grease 2 large baking sheets. Unravel the pastry, brush with milk and scatter half of it with the cheese and cashews. Fold the pastry over to sandwich the nut mixture and press down firmly. Cut into 16-18 thin strips along the longest length. Pick up each strip, twist a little, then lay slightly apart on baking sheets. Bake for 10 mins until golden. Lift onto a cooling rack as soon as they come out of the oven to stop them from sticking to the trays. They’ll last for a week in a tin. Return them the oven for a couple of mins to heat through.
To make the soup, fry the onion in the butter in a large pan for 5 mins until starting to soften. Add the potato, then pour in the milk and stock. Bring to the boil then cover and lower the heat. Season and leave to simmer for 10 mins or until the potatoes are cooked.
Pack the watercress into a blender or large food processor and pour over the hot potato mixture. Blend until smooth then return to the pan to reheat. Serve each portion in small bowls or cups with 2 of the pastries. The remainder can be eaten with drinks on another occasion.