Puree the trout fillets and crème fraîche. Season with salt, pepper and lemon juice. Wash the dill, dab dry and pluck flags from the stems. Drain the cranberries. Spread trout mousse on the pumpernickel thalers and garnish each 4 thalers with cranberries, dill flags and trout caviar.
Clean, wash and drain the lamb's lettuce. Wash, dry and quarter the tomatoes. Season vinegar with salt, pepper and sugar. Gradually add the oil. Arrange pumpernickel, lamb's lettuce and tomatoes.
Drizzle salad with vinaigrette.