Heat 1 tbsp oil in a large flameproof casserole or saucepan. Season the chicken thighs with salt and pepper and brown them on both sides on a medium heat. Remove the chicken to a plate.
Keep the pan on the heat and add the carrots, celery and leeks, along with another tablespoon of oil. Cook over a gentle heat for about 5 minutes until starting to soften. Stir often and use the spoon to scrape and loosen anything left from frying the chicken.
Add the thyme, rosemary and bay to the pan. Cook for 2 minutes then pour in the wine. Increase the heat and let the wine bubble and reduce by half before tipping in the stock and returning the chicken thighs to the pan. Season with salt and pepper and bring to a gentle simmer. Cook gently for 15 minutes.
Add the celeriac and lentils to the pan and continue to gently simmer for about 25 minutes until the lentils are just cooked and the chicken is tender. Lift the chicken out of the pan and keep it to one side. Stir in the kale leaves and simmer gently for a final 5 minutes before turning off the heat, popping on a lid and leaving it to stand for 10-15 minutes while you prepare you chicken and your gremolata.
Add the lemon zest, garlic and parsley to a pestle and mortar along with the walnuts and a pinch of salt. Smash it all together into a coarse paste and stir in a couple of tablespoons of olive oil, enough to give you a pesto-like consistency.
Remove and discard the skin from the chicken and pull the meat away from the bone. Shred the meat with your fingertips. Stir the meat back into the pan. Taste and tweak the seasoning with salt, pepper and a squeeze or 2 of lemon juice to your liking. Remove the bay leaf. Serve in bowls, finished with a spoonful of the walnut gremolata.