Prepare the béchamel (white sauce), by melting the butter in a pan over a low heat and adding the flour.
Then add the milk and, keeping the pan on the heat, continue to stir until you get a thick sauce. Season with salt and pepper. Clean the leek, chop the white part into thin round slices and fry with the knob of butter, adding a very small amount of water. Add salt and cook for around 15 minutes. Cut the Taleggio into small cubes. Roll out the puff pastry to a thickness of around 3 mm. Use a pastry cutter to cut out discs, each with a diameter of approx. 9-10 cm. Brush half of the discs with the beaten egg and place a level spoonful of béchamel sauce, a few Taleggio cubes and a little stewed leek on top. Cover each filled disc with another puff pastry dish, ensuring the edges are well attached. If necessary, seal the edges by pressing down gently with the prongs of a fork.
Brush the upper part with the beaten egg. Place the shapes on an oven sheet lined with baking paper and bake in a hot oven at 200°C for 10 minutes.