Preheat the oven to 190 degrees C. Make a sauce by whisking together half the butter with the flour and milk in a saucepan over moderate heat, until it comes to the boil and thickens. Simmer for 1 minute, then pour into a bowl. Drain the salmon and flake the flesh into the sauce. Spoon out half the asparagus tips (keep the rest for another dish), drain and add to the sauce. Snip in the dill; season generously with salt and pepper.
Melt the remaining butter. Place the sheets of pastry on top of each other, cut out four 18cm squares and brush with melted butter. Spoon a quarter of the salmon mixture into the centre of each square and draw up the corners to make a bundle. Brush the outside with butter and place on a greased baking tray. Bake for 12–15 minutes, until golden.