Combine chicken and stock in large pot. Bring to boil and skim off the foam. Reduce heat to low and simmer,covered for 45 minutes. Strain chicken and reserve 6 cups of stock. Cool the chicken and shred meat. Set aside. Finely grind walnuts, garlic, cilantro and pepper with salt in food processor. Set aside. Melt butter in dutch oven over medium heat. Add onions and saute until they just begin to color about 8 minutes. Stir in flour and cook stirring for 1 minute. Gradually stir in reserved chicken stock, simmer for 5 minutes. Turn heat to low and stir in walnut mixture. Let simmer for 5 minutes. Whisk egg yolks in small bowl then add a ladleful of hot broth. Stir and add back to the chicken stock. Add all spices, tarragon and salt and simmer without boiling for 10 minutes. Take off heat and stir in vinegar and chicken. Serve garnished with cilantro. Better the next day.