To spatchcock the chicken, set the bird on your worktop, breast-side down, legs towards you. Using good kitchen scissors, cut through the flesh and bone along both sides of the backbone. Remove the backbone and keep it for stock (freeze until you’ve gathered three or four). Open the chicken, turn it over, then flatten it by pressing hard on the breastbone with the heel of your hand. You’ll feel it breaking and flattening under your hand. Remove any big globules of fat and neaten any ragged bits of skin. Now you have a spatchcocked bird.
Using a pestle and mortar, pound the garlic to a purée with some sea salt flakes. Add the chilli, oregano, extra virgin olive oil and some pepper. Put the chicken in a dish or roasting tin that will fit in your fridge. Gently loosen the skin of the breast, pushing your fingers in between the skin and the flesh. Work your way under the skin, over the legs too, and spoon some of the garlic and herb mixture in under the skin, then spread it all over the chicken, on both sides. Cover with cling film and put in the fridge for a few hours.
Mix the egg yolk, mustard and crushed garlic together in a bowl. Using electric beaters or a whisk, start gradually adding the oils in little drops. Make sure that each drop has been incorporated before you add the next. If your mixture splits, just start again with a new egg yolk, and gradually add the curdled mixture to it. Add the chipotle paste, lemon juice to taste and seasoning. Cover and keep in the fridge for up to 4 hrs.
Heat oven to 180C/160C fan/gas 4. Lay the chicken in a roasting tin, breast-side up. Season all over and roast for 1 hr, basting a few times during cooking. To check that the chicken is cooked properly, pierce the bird between the leg and the body – the juices should run clear. Cut into serving pieces and serve with the chipotle aïoli. Roasted sweet potato wedges and a green salad are brilliant on the side. Any leftover aïoli will keep covered in the fridge for two days.