Heat the oven to 190C. Put one onion inside the cavity with one sprig or rosemary.
Rub the skin with a little olive oil, then season with salt and pepper. Put the chicken, breast-side down, in a large roasting tin, then roast for 45 mins.
Meanwhile peel and cut the onion into wedges. Scrub, then halve the potatoes. Peel and cut the squash into chunks. Slice the chorizo into chunky rings. Cut the chilli in half. Bash the garlic with a meat tenderizer.
Take the chicken out of the oven and turn it over. Scatter all the prepared veggies and chorizo around the chicken, toss in the pan juices and some extra olive oil. Season to taste then roast for about 50-60 minutes until the chicken is golden.
Take the chicken out of the tray and rest it on a plate covered with foil and tea towels. Cook the veggies further if they are not ready.
Carve and serve with the veggies and the pan juices.