Scrub, peel and halve the potatoes, quartering any larger ones. Add to a large roasting tray.
Break the garlic bulb into cloves, leaving them unpeeled, then lightly crush with the flat side of a knife. Pick the rosemary leaves, discarding the stalks. Add the garlic and rosemary leaves to the tray.
Drizzle with oil, season with sea salt and black pepper, then toss well and spread out in an even layer.
Rub the chicken all over with a pinch of salt and pepper and a drizzle of oil. Stuff the chicken cavity with the whole lemon and the thyme sprigs. Place the chicken in the tray, on top of the vegetables.
Reduce the oven temperature to 200ºC/400ºF/gas 6, then add the chicken and roast for 45 minutes.
Carefully remove the tray from the oven, use tongs to turn the vegetables over, then spoon any juices from the tray over the chicken.
Return the tray to the oven for a further 30 minutes, or until the chicken is cooked through. To check, pierce a chicken thigh with the tip of a sharp knife – if the juices run clear, it’s done. Otherwise return the tray to the oven, cook for a little while longer and repeat the test.
Once cooked, transfer the chicken to a board and return the vegetables to the oven for a final 5 minutes to crisp up, if needed.
Cover the chicken with a layer of tin foil and a tea towel, then leave to rest for 10 to 15 minutes.
Using a sharp carving knife, carve up the chicken, then serve with the roasted veg. Delicious with a green salad on the side.
