Heat oven to 190C/170C fan/gas 5. Stuff the chicken cavity with 2 lemon halves, 2 rosemary sprigs, 4 onion quarters and some seasoning. Put the chicken into a large roasting tray.
Crush 3 garlic cloves, then add to a small bowl with the thyme leaves and olive oil, mix, then rub this all over the chicken with some seasoning.
Arrange the potatoes around the chicken with the rest of the lemons and onions, rosemary and whole garlic cloves, and lemon thyme. Cook in the oven for 1 hr 20 mins, turning the potatoes in the pan once or twice. To check if the chicken is cooked, pierce the thigh with a skewer and the juices should run clear.
Remove the chicken from the tray and place on a serving dish or board to rest for 15-20 mins. As you lift the chicken, let any juices pour back into the tray. Increase oven temperature to 200C/ 180C fan/gas 6 and cook the potatoes for a further 10-15 mins or until tender.
If you want gravy, use a slotted spoon to remove the potatoes from the tray to a serving bowl, then use a potato masher to mash all the juices, lemons, herbs and garlic together in the roasting tray, adding a splash of water if it’s too pulpy. Push this mix through a sieve, taste the gravy and add some more seasoning if you need to. Bring everything into the garden so everyone can help themselves.