Line 4 150 ml (¼ pint) pudding moulds or china ramekin dishes with clingfilm, then line the bases and sides with the sliced trout, keeping a little back for putting on the tops.
Mix the crème fraîche, lemon rind and lemon juice with a little black pepper. Flake the fish, discarding any skin and bones, then fold it into the crème fraîche mixture. Divide between the lined moulds then cover with the remaining sliced trout. Fold any overhanging pieces of trout over the top then chill until required.
To make the salad dressing, toast the walnuts in a dry frying pan over a medium heat until lightly browned. Mix the oil, vinegar and a little seasoning together then add to the hot walnuts and set aside.
When ready to serve, tear any very long stems off the watercress then toss with the dressing. Unmould the mousses, remove the clingfilm and arrange on 4 small plates. Spoon the salad around and serve with warmed crusty brown rolls, if liked.