The quality of the walnuts and spices is crucial for Satsivi. The sauce should be thick enough to form a thin, creamy layer on a spoon, not runny. The dish must be served cold for the sauce to set and the flavours to meld. It is traditionally prepared a day ahead.
Place the chicken or its parts (thighs, breasts) in a pot, cover with water, add the whole onion and salt. Bring to a boil, reduce the heat, and simmer covered until the chicken is tender (about 60-90 minutes).
Remove the chicken, let it cool, and cut (or shred) it into medium-sized pieces. Strain the strong broth and set aside (you will need about 1 litre).
Finely grind or blend the walnuts. The walnuts should be ground until oil is released when the paste is pressed. Reserve this oil for the final garnish.
Mix the ground walnuts with the minced garlic and all the spices (Khmeneli Suneli, clove, saffron, salt) to form a paste.
In a large pot or deep skillet, heat a little oil (or butter) and sauté the finely chopped onion until soft and golden (about 10-15 minutes).
Add the flour to the onion, stir, and cook for one minute.
Gradually pour 1 litre of hot chicken broth into the onion and flour mixture while stirring constantly. Cook until the sauce thickens. Add the wine vinegar.
Remove the sauce from the heat and stir in the prepared walnut paste (without the reserved oil). Mix well until the sauce is smooth and creamy. If too thick, add a little more broth; if too runny, let it simmer uncovered for a bit.
Add the chicken pieces to the sauce. The mixture should completely cover the chicken. Transfer to a serving dish, let cool completely, and then refrigerate for at least 12 hours.
Before serving, garnish with the reserved walnut oil (if available) and ground red pepper.