Wash the salmon fillet and dab dry. Halve 1 lemon and squeeze the juice. Sprinkle fish fillets with it. Peel the lower third of the asparagus spears and cut off the woody ends.
Wash the asparagus. Cook in plenty of boiling salted water with a little sugar for 10-12 minutes at medium heat. Then drain and keep warm. Bring cream and 1/4 litre of asparagus water to the boil.
Knead the fat and 1 tablespoon of flour into a smooth dumpling. Stir pieces into the sauce. Let the sauce simmer for about 5 minutes, stirring from time to time. Add mustard and season the sauce with salt, pepper and sugar.
Turn the salmon fillets in the remaining flour. Heat the oil in a pan and fry the fillets on each side until golden brown. Wash the remaining lemon and cut into slices. Arrange asparagus, salmon fillets and mustard sauce on a plate.
Serve garnished with lemon wedges and parsley. New small jacket potatoes taste good with it.