Heat oven to 200C/180C fan/gas 6. Pour the olive oil into a large roasting tin and add the potatoes, olives, lemon quarters, bay leaves and garlic. Toss everything together so it’s coated in oil and evenly distributed. Add the chicken thighs, skin-side up, and season.
Put the roasting tin in the oven and roast for 1 hr, basting with the pan juices halfway through cooking. After 1 hr, check that the potatoes are soft and the chicken is cooked through, then return to the oven for a final 15 mins to crisp the chicken skin.
Remove the roasting tin from the oven. Press down on the roasted garlic cloves with the back of a spoon, discard the skins, and mix the mashed garlic with the meat juices. Serve with watercress or your favourite salad leaves on the side.