Fish:
Clean the salmon fillet and remove any bones if necessary.
Coarsely crush the peppercorns and mustard seeds in a mortar. Mix in the lemon zest and two-thirds of the olive oil to make a marinade.
Rub the marinade over the fillet and press it onto the fish.
Heat the remaining oil in a pan over medium heat. Place the fillet in the pan with the peppered side down. You can add some vegetables to the pan if desired.
After about 2 minutes, flip the fillet and let it sit in the pan for another 2–3 minutes, leaving the inside slightly translucent (glassy).
Carefully slice the salmon into portions using an electric knife and season with a bit of salt.
Sauce:
Mix mustard, vinegar, and sugar together.
Slowly drizzle in the olive oil, whisking continuously (similar to making mayonnaise), to form a smooth sauce.
Stir in the chopped dill.