1. Preheat the oven to 180C/350F/Gas 4.
2. Heat the oil on the hob over a high heat in a wide, shallow, ovenproof and flameproof casserole (that will ultimately fit all the chicken in one layer, and that has a lid). Sear the chicken, 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.
3. 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.
4. Put 20 of the unpeeled cloves of garlic (papery excess removed) into the casserole, top with the chicken pieces, skin-side up, and then cover with the remaining 20 cloves of garlic.
5. Add the vermouth (or white wine) to any oily chicken juices left in the bowl. Swill it around and pour this into the casserole.