Preheat the oven to 180°C / gas mark 4. Heat the oil on the hob in a wide, shallow ovenproof and flameproof casserole (that will ultimately fit all the chicken in one layer, and that has a lid), and sear the chicken over a high heat, skin-side down. This may take two batches, so transfer the browned pieces to a bowl as you go.
Once the chicken pieces are seared, transfer them all to the bowl. Finely slice the spring onions, put them into the casserole and quickly stir fry them with the leaves torn from a few sprigs of thyme. Put twenty of the unpeeled cloves of garlic (papery excess removed) into the casserole, top with the chicken pieces skin-side up, then cover with the remaining 20 cloves of garlic. Add the vermouth (or white wine) to any oily, chickeny juices left in the bowl. Swill it around and pour this into the casserole.
Sprinkle with the salt, grind over the pepper, and add a few more sprigs of thyme. Put on the lid and cook in the oven for 1 1/2 hours.
Make ahead note:
Chicken can be browned and casserole assembled one day ahead. Cover tightly and store in fridge. Season with salt and pepper and warm casserole gently on the hob for five minutes before baking as directed in recipe.
Recipe from Nigella: Kitchen by Nigella Lawson (published by Chatto & Windus).