Preheat oven to 200 degrees.
Over a gentle heat, melt the butter in a medium saucepan, add the flour and stir until smooth. Using a whisk, add the milk stirring continuously until thoroughly mixed and there’s no lumps. Increase heat to medium and cook the mixture for 3 minutes, keep stirring. Add the salmon, dill, capers, and spring onions. Remove from heat, season to taste and set aside to cool.
Remove the puff pastry from the package and allow to soften. Place one piece of the pastry into a lightly greased 25cm pie dish. Gently pour the milk and salmon mixture onto the pastry. Cover the pie with the second piece of puff pastry dough. Fold the edges over to seal and crimp the edges. Brush the top of the pastry with beaten egg then bake for 30 minutes until the pastry is golden.