Heat the oven to 190C/170C/gas 5. Brush a quarter of the butter over a medium casserole dish, arrange a layer of potatoes in the bottom of the dish, then scatter with a little parmesan, thyme, a drizzle of the cream and salt and pepper. Continue until you’ve used up all the potato slices, most of the thyme (reserve some to serve, if you like) and about half the cream, then pour the rest of the cream over the final layer.
Season the chicken with salt and pepper and put the thyme stalks, garlic and bay in the cavity, then rub over the remaining butter. For the best results, sit a roasting rack or ovenproof cooling rack over the gratin dish and sit the chicken over the gratin so the juices can drip into the potatoes. If you don’t have a roasting or cooling rack, simply sit the chicken in the middle of the gratin. Roast for 1 hr 20 mins until the chicken skin is golden and it’s cooked through. To check, pierce the thigh with a skewer – the juices should run clear.
When the chicken is ready, remove everything from the oven and turn it up to 200C/180C fan/gas 6. Use tongs
to lift the chicken onto a board, allowing any juices from the cavity to drizzle over the potatoes as you lift it. Remove the rack, if using, and use a spatula to press the potatoes down, then return to the oven for 10 mins to brown while the chicken rests. Remove the potatoes from the oven, scatter over the reserved thyme, if you like, and leave for 5 mins before serving alongside the chicken. Serve with leafy greens like cabbage or kale.