Drain the herb butter in a large pan, dust with flour, sweat and gradually deglaze with fish stock while stirring constantly, fold in the mustard, deglaze with cream and simmer for about 10 minutes.
Dissolve the sugar, fold in the dill, season with salt and pepper and stir in the lemon juice, put the salmon pieces in the sauce and leave to stand on a low flame for about 10 minutes with the lid closed.