Heat oven to 170°C/ gas 5. Place 2 lemon halves, 2 sprigs of rosemary, 4 onion quarters and some seasoning in the cavity of the chicken. Put the chicken into the roasting tray.
Crush 3 garlic cloves, add to a bowl together with the thyme leaves and olive oil, combine well and then rub all over the chicken with salt and pepper.
Place the potatoes around the chicken with the remaining lemons and onions, rosemary and whole garlic cloves, and sprigs of lemon thyme. Place in the oven to cook for 1 hr 20 mins, turning the potatoes in the pan once or twice. To check if the chicken is cooked, use a skewer to pierce the chicken, the juices should run clear.
Take the chicken from the tray and place on a serving plate to rest for 15-20 minutes. Take the chicken out and then let any juice pour back into the tray. Turn up the temperature of the oven to 180°C/gas 6 and cook the potatoes for a further 10-15 minutes until soft and crispy.
To make the gravy, use a slotted spoon to lift the potatoes from the tray and tip into a bowl. Use a potato masher to mash all the juices, lemons, herbs and garlic together in the roasting tray, adding a splash of water. Sieve, taste the gravy and add some more seasoning if desired.