Flake the smoked trout into bite-sized pieces, removing any bones, and place in a large salad bowl.
Wash and drain the grapes and watercress and add them to the bowl. Finely slice the fennel and add to the mix.
Make the dressing by mixing the mayonnaise, cornichons, capers and lemon juice. Season to taste with salt and pepper, then carefully mix through the salad and serve.