Scrub potatoes and pierce each with a fork.
Set potatoes directly on the rack in a 325° oven and bake until soft when pressed, 1 to 1 1/4 hours (about 45 minutes in a convection oven).
Meanwhile, in a blender or food processor, combine lemon juice, parsley, watercress, tarragon, and sour cream. Whirl until mixture is smoothly puréed, scraping container sides as required.
Cut a slit across the top of each potato and pinch sides to pop open top. Set potatoes on a platter and spoon a little of the green sauce into each. Serve with remaining sauce and salt to add to taste.