Pour a splash of the oil into a large, round casserole dish and rub it all over the base and sides. Heat the oven to 220C/200C fan/gas 7. Slice the potatoes as thinly as you can (no need to peel) and arrange in layers with the garlic in the base of the oiled dish. Remove any string from the chicken, drizzle over the remaining oil and rub in well. Season the chicken all over generously, including inside the cavity, then add one of the lemon halves inside the cavity. Put the chicken on top of the potatoes, then pour the vermouth and stock around the potatoes. Roast for 40 mins, until the chicken is golden brown.

Meanwhile, tip the peas into a blender or food processor with the tarragon, nuts, cream and Dijon mustard. Pulse a few times until everything is finely chopped, then season with salt and pepper. Remove the chicken from the oven and pour the sauce directly over. Reduce the oven temperature to 180C/160C fan/gas 4 and return to the oven. Roast for a further 20-30 mins until the chicken is cooked through. To check, pierce the thickest part of the chicken with a sharp knife. The juices should run clear, or a temperature probe inserted should read 74C.

Remove the chicken from the tin, then cover and leave to rest for 10-15 mins before carving. Pour any resting juices over the potatoes and season with salt, pepper and the remaining lemon juice. Serve with a green salad, if you like.