Heat the oil in a large casserole dish / dutch oven over a medium heat. Add the chopped leeks, celery, carrots, onions, and garlic and stir. Add 1 tbsp of the fresh chopped thyme and 1 tbsp of the fresh chopped tarragon. Saute for 5 minutes or so until the vegetables have started to soften slightly.
Add the stock and bring to the boil. Once boiling add the cubed chicken, a pinch of salt and some grated black pepper. Lower the heat to a slow simmer and cover. Leave simmering for about 8 minutes until the chicken has just cooked through.
Whilst the stew is simmering, combine the flour, baking powder, butter, pinch of salt and the remaining chopped herbs into a bowl. Cut the butter into the flour mixture until crumbly. Make a well in the middle to the mixture and pour in the milk. Combine till you get a wet dough.
You well get approx 8 dumplings out of this mixture. Drop dessert spoons of the dough into the stew (each dumpling will be about 1 small dessert spoon of mixture). Cover the casserole dish / dutch oven and simmer for 6 - 8 minutes or until the dumplings have puffed up. Remove from the heat and serve or...
If you want your dumplings browned remove from the heat and place uncovered under a medium / hot grill for 2 - 3 minutes until they have turned golden brown.