Set the oven to Gas Mark 4, 180°C (350°F).
To make the stuffing: Melt the butter in a good sized saucepan. Fry the onion for 3-4 minutes until it begins to soften then add the chorizo and cook for another minute or two. Place the breadcrumbs in a medium sized bowl. Remove the onion and chorizo from the heat and add them to the bowl with the breadcrumbs. Add in the herbs and seasoning. Stir well to mix. Allow the stuffing to cool before packing it into the body cavity of the chicken. Fold over any loose skin to close. Secure with a cocktail stick.
To make the spice mix: Mix the smoked paprika, garlic granules, cumin, salt, a little pepper and a tablespoon of oil in a small bowl. Then brush this over the body and legs of the chicken. Place in a roasting tin. Add the chopped onion to the tin then pour in 150ml of the water or stock. Reserve the rest for the gravy. Cover loosely with tin toil and place in the preheated oven.
After 1 hour, remove the foil from the chicken, baste with the roasting juices. Roast, uncovered, for another 20 minutes until the skin is golden brown and the juices from the thighs run clear.
Remove from the oven, place the chicken on a carving board and keep warm while you make the gravy.
To make the gravy: Skim all but 1 tablespoon of fat from the juices in the roasting tin, then sprinkle in a tablespoon of flour. Cook for a couple of minutes and then add in the remaining 350mls stock or water and bring to the boil, stirring and scraping to loosen the caramelised meat juices. Simmer for 2-3 minutes and season to taste. Strain into a gravy boat, discard the vegetables and serve.