Clean the asparagus and trim any white ends or peel with a vegetable peeler.
Preheat the oven to Gas mark 5, 190┬║c, 375┬║f.
Heat the butter in a large pan, then cook the flour for 1 minute; remove from the heat. Gradually whisk in the milk and cook, stirring, until thickened and smooth and season with black pepper.
Remove from heat and stir in the dill, chives lemon juice and zest and half the Parmesan. Check the seasoning.
Spoon and spread a little of the sauce over the base of a 67 oz ovenproof dish and arrange a layer of lasagne sheets, overlapping, on top. Scatter over a third of the salmon and asparagus, followed by some of the white sauce. Continue to layer up, finishing with pasta topped with sauce. Sprinkle over the remaining chives and Parmesan.