Put the salmon fillet in a shallow pan in which it fits snugly and add the stock, the stems from the dill (reserve the fronds for the sauce), the peppercorns and lemon juice. Bring to the boil then simmer gently for 8–10 minutes until barely cooked. Remove the fish from the stock and, when cool enough to handle, break up into coarse flakes. Strain the stock and discard the flavourings. Set aside.
Steam the trimmed asparagus for about 3 minutes until barely tender. Cut off and reserve the tips then chop the stalks into short pieces. Chop the reserved dill fronds.
To make the sauce, melt the butter in a saucepan and stir in the flour then gradually stir in the reserved stock and the milk. Bring to the boil, stirring constantly, to make a smooth light sauce. Simmer for 1 minute then remove from the heat. Add the lemon zest, the chopped dill fronds, sour cream and seasoning to taste. Stir in the salmon and chopped asparagus stalks.
Arrange enough sheets of lasagne to cover the bottom of a greased lasagne dish or large shallow ovenproof dish of about 1 1/2 litre capacity. Spoon over 1/4 of the salmon mixture. Repeat the layers twice more, then cover with the remaining lasagne sheets. Top with the reserved asparagus tips and press them down gently all over. Top with the remaining salmon mixture.
Sprinkle with Parmesan, then bake for 30–35 minutes until the mixture is bubbling and the top is golden.