Fill the mustard with the egg yolk in a wide bowl and stir. Season the mixture with salt, season with pepper and add the juice of one lemon. Beat heartily with the mixer or the whisk, while leisurely pouring in the soybean oil. Fold the shallots and dill into the sauce and season a second time. Loosely divide the salmon slices evenly among 4 plates and baste with the sauce. To serve: Wholemeal toast and butter.