Fillet the salmon trout, but do not skin it, and pull out the side bones with tweezers. Separate the two fillets cleanly. Place one fillet skin side down in an appropriately sized dish.
Mix salt with sugar and pepper and sprinkle over the fillet. Coarsely chop the dill with the stalks and sprinkle over the top. Drizzle with cognac and season with the ground coriander and pepper.
Place the second fillet, seasoned in the same way, skin side up on top. Cover with foil and marinate in the fridge for at least 48 hours. During this time, turn the fillets twice.
Using a fillet knife, cut the fillets into diagonally thin slices. Serve with 2-3 small red onion rings, a lime or lemon fillet.
The marinated fillets can also be frozen after the marinating process, well wrapped or, better, sealed.
A little mustard sauce and small warm potatoes are an excellent accompaniment.