To spatchcock the chicken, flip it over so the backbone is facing you. Using a sturdy pair of kitchen scissors, cut down either side of the backbone, then discard. Turn the chicken over and push down firmly on the breastbone to flatten it out, then make a few slashes in each leg joint. (Or you can ask your butcher to spatchcock the chicken for you.) Put the chicken in a large sealable plastic bag, or a dish.
Next, make the marinade. Rinse the preserved lemon under running water to remove some of the brine, then separate the middle from the peel (the middle part – the flesh and pips – is very sour) and discard the middle. Finely chop the soft peel until it resembles a paste. Tip into a bowl and add the remaining marinade ingredients and a pinch of salt. Whisk until combined, then pour roughly half the marinade over the chicken and rub it into all the nooks. For the best flavour, leave the chicken to marinate overnight, covered, in the fridge (store the remaining marinade in the fridge too), but if you’re short on time, 2 hrs marinating will do.
Heat oven to 200C/180C fan/gas 6. Tip the onions and beetroot into a large roasting tin (use your best tin and you can serve the dish straight from it), drizzle with oil and season well. Toss the vegetables to coat and spread them out towards the edges of the tin. Place the chicken on top, skin-side up, and pour over any juices from the bag or dish. Season the chicken well and roast for 45 mins.
Meanwhile, bring a pan of water to the boil and add the spelt. Cook for 20 mins or until tender, then drain and set aside. Halve each avocado, remove the stones and peel. Chop each piece in half again, if you like. Char the avocado pieces on a hot griddle pan for a few mins – this will give it a smoky flavour.
Remove the remaining marinade from the fridge and very gently toss the avocado pieces in it, then set them aside. Pour the remaining marinade into a small pan and bring to a rapid boil, then continue cooking for 2-3 mins until you have a sticky glaze.
Remove the roasting tin from the oven, lift the chicken onto a plate or board and stir the spelt into the vegetables. Place the chicken back on top, dot the avocado pieces around the tin and brush the thickened glaze over the chicken. Return to the oven for a further 5-10 mins. To check the chicken is cooked, pierce the thickest part with a skewer. The juices should run clear, not pink. If you need to, return the chicken to the oven for a further 5-10 mins, then check again.
Lift the chicken onto a plate, cover loosely with foil and return the roasting tin with the vegetables to the oven for a further 10 mins until the avocado starts to soften. To serve, nestle the chicken back into the tin of veg, scatter over a few small watercress leaves and serve the rest on the side with the yogurt.