For the chicken:

1. Put the chicken and the stock in a large soup pot. Bring to a boil. Reduce the heat to low and simmer, covered, until the chicken is cooked, about 45 minutes. Remove the chicken with a slotted spoon and reduce the chicken stock to 6 cups. If using store-bought chicken stock, do not reduce, just set aside 6 cups.
2. Cool the chicken until manageable, then remove and discard the skin and bones and shred the meat into medium (not quite bite size) pieces and set aside.

For the sauce:

1. Finely grind the walnuts, garlic, cilantro and chile with the coarse salt in a food processor. You will have to do this in batches. Set aside
2. Melt butter in a large pot over medium heat. Add the onions and sauté until they just begin to color 8 to 10 minutes. Stir in the flour and cook, stirring for 1 minute more. Gradually stir in the chicken stock. Let simmer for 5 minutes without boiling.
3. Turn the heat down to low and gradually add the ground walnut mixture, stirring with a wooden spoon. Let simmer for 3 to 4 minutes.
4. Whisk the egg yolks in a small bowl, then stir in a ladleful of the simmering mixture. Whisk the yolk mixture back into the sauce.
5. Add all of the spices, the tarragon and salt and let simmer without ever allowing the sauce to boil, about 10 minutes. Off the heat stir in the vinegar. Add the reserved chicken to the sauce and stir gently to coat. Cool to room temperature, cover and refrigerate overnight.

To serve:

1. Remove the Satsivi from the refrigerator about 30 minutes before serving and gently stir. Place the Satsivi in a large serving bowl and garnish with walnut pieces and cilantro sprigs. Serve with a large basket of pita bread triangles, and bowls of sliced green onions.