Preheat oven to 400 degrees F.
Wrap beets tightly in foil and roast in middle of oven 1 to 1 1/2 hours, or until tender.
Unwrap beets carefully and cool until they can be handled. Discard stems and peel beets.
Beets may be prepared up to this point 1 day ahead and chilled, covered. Cut each beet into 8 wedges and cover.
To make vinaigrette: In a blender, blend together mustard, vinegar, tarragon, salt, and pepper. With motor running add oil in a stream and blend until emulsified. Vinaigrette may be made 1 day ahead and chilled, covered.
Make herbed goat cheese: In a bowl, stir together bread crumbs, tarragon, salt and pepper. Cut each cheese round in half crosswise. Coat each piece of cheese evenly with crumb mixture, pressing gently, and transfer to a baking sheet. Goat cheese may be prepared up to this point 1 day ahead and chilled, covered loosely. Let cheese come to room temperature before proceeding.
Preheat broiler. Broil goat cheese about 2 inches form heat until crumbs are lightly browned, about 2 minutes.
In a bowl, toss watercress and onion with half of vinaigrette. Arrange watercress mixture, beets, and goat cheese on 8 salad plates and drizzle remaining vinaigrette over beets.