Preheat the oven to 180°C/160°C Fan/350°F. Untruss the chicken, if it comes trussed, and remove all the string. Prepare the vegetables: peel and cut the carrots into batons; cut the trimmed leeks into smallish rounds, approx. 1.5-2.5cm / ¾-1 inch.Heat the oil in a large heavy-based casserole/ Dutch oven with a tightly fitting lid. I use an enamelled cast iron oval casserole 29cm long, in which the chicken fits neatly, leaving just a small space all around it to fit the vegetables later. Place the chicken in the hot oil breast side down to colour the skin; I do this over high heat for about 3 minutes, until the skin is richly golden. (If you’re not using a heavy based casserole, you will need the heat lower.) Then turn the chicken the right way up.Turn down the heat, or take the pan off the heat completely, and aiming for the space around the chicken, add the lemon zest and minced garlic (obviously some can end up on the chicken itself) and give it a quick stir into the oil as best you can. Scatter in the prepared vegetables around the chicken, followed by the salt, chilli flakes (if using) and dried tarragon.Pour in the cold water, but just add 1.25 litres / 5 cups first, and then, gradually, pour in the remaining water until the liquid comes up about two thirds of the leg of the chicken, leaving the golden breast clearly untouched by a small margin. Now add the lemon juice.Turn up the heat and bring the pot to a boil, leaving it uncovered. Poke the vegetables down with a fork if they bob up too much.Once boiling clamp on the lid and put into the preheated oven for 1 hour 15 minutes.Take the pot out of the oven and stir in the orzo, around the edges of the chicken, and then put the lid on again, and put the casserole back in the oven for another 30 minutes, by which time the orzo should be soft and swollen.Take out of the oven and remove the lid, then let it stand for 15 minutes, giving the orzo a bit of a stir to loosen any that has stuck to the bottom of the pan. The orzo will continue to soak up some of the broth as it stands.Stir in 4 tablespoons of the freshly chopped parsley, and then sprinkle over the remaining 2 tablespoons, and put more on the table to serve alongside. You could add the parsley once you’ve shredded the chicken, but it looks so wonderful in its pot, I love to bring to the table whole.To serve, put a small dish by the casserole, then strip off the meat from the chicken, removing the skin and bones to the dish. (For me, these bits are a particular treat: I live for the cartilage!) I find it easiest to do this while the chicken’s still in the pot, but it you prefer you can try and remove it to a carving board; go carefully as it’s likely to fall to pieces a bit as you do so. Stir the meat and orzo again before serving, adding more parsley as you do so, or let grateful eaters add their own as they wish. You may also want to offer parmesan to grate over: I prefer it without it, but there is a strong pro-parmesan contingent in my house.