Heat the oven to 200c/fan 180c/gas 6. Roll the pastry out to the thickness of a £1 coin on a floured surface and divide in 2 (divide the pastry first if you are short on space).
Lift 1 piece onto a buttered baking tray, spoon the trout mix into the middle and spread to a 10 × 18cm rectangle.
Wet a border around the filling, put the other piece of pastry on top, press down around the filling and trim the edges (use these for decoration).
Brush with glaze, cut slits in the top and bake for 30 minutes.