Drain the fish and cut into pieces. Stir yoghurt, lemon juice and crème fraîche until smooth. Season to taste with salt, pepper and sugar. Clean, wash and drain the salads.
Pluck lettuce except for lamb's lettuce in pieces. Clean, wash and thinly slice the mushrooms, mix vinegar, salt, pepper and sugar. Whip the oil into the mixture. Draw salad and mushrooms through the marinade and arrange on plates.
Arrange fish and sauce on top. Garnish with dill and lemon. Serve with baguette.