- 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 a 4-quart pot, bring 2 quarts 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 Y4-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 for seasoning.
- Serve in a warmed tureen, sprinkled with tarragon.