In bowl mix together the butter, 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 onto 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 + 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 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 Mild Curry Paste and continue to cook until the gravy has thickened. Check the seasoning.