Preheat the oven to 200°C, 400°F, Gas Mark 6. Meanwhile, heat the oil in a frying pan and fry the onion for a few minutes until softened. Spoon the onion into a bowl and add the Herbes de Provence, sausagemeat, purée and cheese. Season with salt and pepper and mix well.
Roll the pastry out on a lightly floured surface to a rectangle measuring 28 x 24 cm. Cut the pastry in half lengthways. Divide the sausage meat mixture in half and spoon down the centre of each pastry rectangle and then fold the pastry up and join the edges together, sealing with a little cold water. Cut each long sausage into 7 pieces.
Lift the sausage rolls onto a baking sheet, placing the joined edge on the tray surface. Brush the sausage rolls with a little beaten egg or milk. Bake for 15-20 minutes, until golden and well risen. Remove from the oven, cool and serve.