Cook the onion and garlic in a saucepan with 15g (1/2 oz) of the butter for 1 minute.
Add the mushrooms and cook for a further 2 minutes. Remove from the heat and stir in the breadcrumbs, basil and cheese.
Brush half of each sheet of pastry with melted butter and fold in half lengthways. Divide the filling between the pastry. Fold over the sides of the pastry and roll up.
Place on a greased baking sheet, brush with melted butter and bake at 190 �C / 375 �F / Gas 5 for 20 minutes, until golden brown. Serve hot.