How to Make It
Cut the tomato in half crosswise, and squeeze the halves into a strainer set over a bowl to release the seeds and juice. Press with a spoon to extract as much juice from the seeds as possible; set aside. Cut the tomato flesh into 3/4-inch pieces. (You will have about 1-1/2 cups.) Put the tomato pieces in a bowl and add the seeds in the sieve. Add the remaining ingredients to the bowl and stir to mix.
Measure the tomato liquid; if necessary, add enough of the tomato juice, Bloody Mary mix or V-8 juice to bring the liquid to 5 tablespoons. Combine the tomato liquid and the remaining ingredients in a bowl, whisking to emulsify the sauce.
At serving time, divide the sauce among 4 plates. Place a 1/2 cup ring mold (or a tuna fish can with both ends removed) in the center of one plate and spoon one quarter of the tomato tartare into the mold. Carefully remove the mold. Repeat this procedure on each of the 3 remaining plates. Sprinkle with the chopped tarragon or chives, decorate with the chive flowers, if desired, and serve.