Use a sharp knife to carefully cut down the back of the chicken, so you can open it out flat. Rub half the tzatziki all over the chicken with the juice of ½ a lemon and good pinch of sea salt and black pepper, then cover and leave to marinate in the fridge for at least 2 hours, preferably overnight. Preheat the oven to 180°C/350°F/gas 4. Peel the onions, then very finely chop half an onion and place in a bowl with the juice of ½ a lemon and a pinch of salt to make a pickle. Quarter the rest of the onions and place in a deep tray or ovenproof pan, then halve and add the remaining lemon. Place the chicken skin side up on top (it should fit snugly) and drizzle with ½ a tablespoon of olive oil. Roast for 1 hour 10 minutes, or until beautifully golden and the leg meat pulls easily away from the bone.
When the time’s up, remove the chicken with half the roasted onions and one jammy lemon half to a board for later. Using tongs, carefully squeeze the other jammy lemon half into the pan of tray juices, then cut off and discard the white pith and finely slice the skin, along with the rest of the onions. Put them back into the pan or tray along with the rice and 600ml of boiling salted water, then cover and cook on the hob on a medium-low heat for 12 minutes, or until tender. Spoon the rice into a serving dish, place the chicken, any resting juices and the reserved roasted onions on top, and spoon over the remaining tzatziki. Drizzle with 1 tablespoon of extra virgin olive oil and serve with the pickle and the reserved cooked lemon half, for squeezing over.