Heat the vegetable oil in a large saucepan over a medium heat. Add the sliced onions and sweat until golden brown, stirring occasionally. Add the ginger and garlic pastes to the pan. Cook for 2 minutes, stirring continuously, then add salt, turmeric and chilli powder and cook for another 5 minutes. Add the potatoes and carrots to the pan. Cook until almost tender before adding the beans, green peas, ground cumin and coriander seeds. Cook until the mixture is quite dry, stirring regularly. Remove from the hob, transfer to a plate and set aside to cool. This can be prepared in advance. Preheat the oven to gas 4, 180°C, fan 160°C. Unwrap the sheet of puff pastry on a chopping board and cut in half lengthways. Ask the children to spoon the vegetable mix along one of the long sides of the pastry. Using a pastry brush, brush the edges of the prepared sheet lightly with water. Lift the pastry from underneath the filling and roll over to seal in the vegetables, pressing the edges together to form a sausage roll shape. The kids will love getting involved at this stage, as it's very hands-on. Cut the roll into approximately 3cm long parcels and place on a baking tray. Dip the pastry brush into the whisked egg yolk and use to brush the top of the parcels. Bake for 12-15 minutes, or until the puff pastry has fluffed up nicely and is golden brown. Serve the vegetable puffs warm.