Preheat the oven to 190°C.
In a deep casserole dish, heat the olive oil and sear the chicken all over until litre the skin is golden.
In the meantime, slice the leek and celery, peel the parsnips & carrots and cut them into large chunks.
Transfer the chicken to a plate.
Add the leeks and celery to the pan and cook for 5 minutes, then add the parsnips & carrots. Pour in the stock. Bring to the boil, put the chicken back and cover with a lid. Put the pan in the oven and cook for 60 minutes.
emove from the oven and add the peas and beans. Return to the oven for 15 minutes. Remove from the oven. Transfer the chicken to a serving dish and let it rest. Stir the spinach into the veggies, put the lid on and let it rest for 5 minutes.
Now make the pistou: put the garlic, spinach, basil, olive oil and 1 Tbsp water in a mini food processor. Whizz until you get coarse paste then season to taste.
Carve the chicken and serve with the vegetable broth, topped with a spoonful of pistou.