Chop the dill into small pieces. Mix with the mustard powder and olive oil. Season the marinade with a little pepper. Apply to the smoked salmon fillet with a cake brush. Cover and marinate for thirty minutes.
In the meantime, peel the yellow peel (zest) from half of the lemon and cut into very fine strips. Blanch in boiling hot water for one minute. Squeeze the juice from the whole lemon.
Melt the butter at low temperature. Cool down to hand heat. Stir egg yolks, fish stock and juice of one lemon in a baking bowl. Beat over a warm water bath with a whisk until the mixture is very creamy and binds very easily. Stir the liquid butter leisurely “in a thread” into the egg cream. Continue beating the sauce briefly and season with salt and a pinch of cayenne pepper. Keep warm.
Using a large, sharp kitchen knife, cut the salmon diagonally into slices about 1/2 cm thick. Very lightly oil and heat a grill pan or skillet. Toast the salmon slices over medium heat for fifteen seconds on each side.
Spoon sauce onto warmed plates, top with salmon fillets and sprinkle with lemon wedges.