Rinse then pat the chicken dry. Place in a bowl.
Stir together the salt, pepper, sugar, cayenne, fennel seeds and tarragon. Add to the chicken. Mix well. Cover the bowl and keep in the fridge for a few hours, preferably overnight.
Heat the cooking oil in a wok or frying pan. When the oil is smoking hot, add the chicken pieces and cook until nicely browned. Note that you are not cooking the chicken through at this point but merely searing it.
Pour off the oil.
Return the pan on the stove and add the butter.
Add the onion, carrot and celery, sprinkle with salt and pepper, and sauté for about a minute.
Put the chicken back into the pan, pushing the vegetables to the sides.
Pour in the yogurt. If you seasoned the chicken correctly, there is no need to add any more seasoning at this point.
Turn the heat to low. Cover the pan. Let the chicken cook in the yogurt for about 20 minutes, after which time, the mixture should be almost dry.
Sprinkle with parsley and serve.