In bowl mix together the butter, Patak's Balti Paste, garlic, coriander and black pepper. Rub the chicken inside and out with the spiced marinade. For a more intense flavour, carefully loosen the skin and rub the marinade on to the breast of the chicken as well. Cover and refrigerate the marinated chicken for two hours or even overnight.
Preheat the oven to 190C/375F/Gas Mark 5. Place the onions, carrots and celery in a large roasting tin and toss in the vegetable oil. Stuff the lemon into the cavity of the chicken and place on top of the vegetables.
Cover with foil and roast for 20 minutes per 500g plus 20 minutes. Remove the foil for the final 30 minutes of cooking time so the chicken is golden, the juices will run clear when the chicken is cooked through. Remove the chicken from the roasting tray and rest.
Place the roasting dish on the hob on a medium heat. Once the onions begin to sizzle skim off any fat on the surface. Stir in the flour and cook for two minutes. Slowly whisk in the hot chicken stock, making sure there are no lumps. Add the Patak's Balti Paste and continue to cook until the gravy has thickened. Check the seasoning. Serve the roast spiced chicken with plenty of Indian style gravy.
Tip: If you do not have time to make your own gravy, you can use store bought gravy. Just add the onions to a pan and skim off any fat before stirring in the Patak's Balti Paste, saute for two minutes before pouring in the store bought gravy and check the seasoning.