To make the vinaigrette, combine the mustard, vinegar, salt, pepper, and chopped tarragon in a bowl. Add the olive oil; mix well with a fork or whisk to make a thick dressing. Set aside.
Trim the celery stalks and thinly slice, using a mandoline if possible. Set aside. (You can also set aside celery leaves for garnish, if desired.)
If using smoked trout, remove the skin from the trout and flake into medium pieces.
Divide beans, celery, and fish among 4 salad plates, tucking some celery slices and fish into the beans. Drizzle with vinaigrette, and garnish with tarragon leaves. Serve at room temperature.
