Blitz the trout, watercress, horseradish, butter, lemon juice, lemon zest and olive oil in a food processor until smooth. Divide half the mixture between two chef's rings, each placed on a separate plate.
Sprinkle over a few walnuts, and top up with the remaining trout mixture. Spoon over the Greek-style yoghurt, chill for a few minutes, then carefully remove the chef's rings. Garnish with watercress.