1 pound veal or beef kidneys, or 1 pound chicken giblets.
1 parsley root.
2 ounces celery root.
1 teaspoon salt plus additional salt to taste.
6 whole black peppercorns, tied in a cheesecloth bag with 2 bay leaves.
3 tablespoons long-grain rice.
3 Brined Cucumbers.
6 teaspoons sour cream.
1 tablespoon finely chopped fresh parsley.
To prepare the kidneys, remove their fat and membrane, cut in half, cover with cold water, and let soak for 6 hours, changing the water every 2 hours. Discard the last water.
While the kidneys are soaking, cut the carrot, parsley and celery roots, and onion into julienne strips. In 4-quart pot, bring 2 quarters of water to a boil. Add the kidneys (or the giblets, if you are using them), julienned vegetables, 1 teaspoon salt, and the peppercorns and bay leaves, and bring to a boil again. Lower the heat and simmer, partially covered, for 30 minutes. Meanwhile, peel the potatoes and cut into 1-inch cubes. Strain the stock, discarding the vegetables. Cut the kidneys into 1/4-inch slices and return to the stock, adding the potatoes and rice. Cook slowly, partially covered, for 20 minutes, then add the pickles and simmer 5 minutes more. Turn off the heat, cover completely, and allow the flavors to mingle for 5 minutes. Blend the sour cream with 1 cup of soup and stir it back into the pot, then taste the seasoning. Serve in a warmed tureen, sprinkled with tarragon.