Skin and bone the salmon and set aside. Preheat the oven to 450 degrees F/230 degrees C.
Cut the tips off the asparagus. Reserve the stems and poach the tips in boiling salted water until tender, 3 to 5 minutes. Drain well, refresh in ice cold water, then drain again, leaving to sit so that all the water comes off. Puree the asparagus and stir in the creme fraiche, dill and lemon zest to blend. Season with salt and pepper and set aside.
Lay 1 sheet of puff pastry on a damp baking sheet with the long edge facing you. Lay the salmon on the pastry, like a picture in its frame. Sprinkle with salt and pepper. Lay the raw stems of asparagus on top of the salmon, like pencils. Spread the asparagus puree over top.
Brush the margins of the pastry with the egg wash. Lay the second sheet of pastry over the salmon and press the edges to seal, like a giant ravioli. Trim the edges, leaving a 1-inch/2.5 cm border. Press with the tines of a fork, then, with the dull edge of a knife, scallop the edges. Make 2 or 3 slits in the top to allow steam to escape. Brush all over with the egg wash and bake until puffed up and golden brown, about 20 minutes.
Remove from the oven and let cool for 5 minutes before slicing. This dish is also good at room temperature.