Tandoori roast chicken

Name: Tandoori roast chicken
Prep time: 15 min
Cook time: 1 h 45 min
Total time: 2 h
Cuisine: Indian
Ingredients: 1.8kg chicken; 2 onions , thickly sliced; 1 lemon , halved; thumb-sized piece ginger , peeled and thickly sliced; 400g can coconut milk; small bunch coriander , roughly chopped; 150ml pot natural yogurt; 1 tbsp tomato purée; juice 1 lemon; 1 tsp each hot chilli powder, turmeric, ground coriander, ground cumin, garam masala and ground cinnamon; 6 garlic cloves , whizzed to a paste with the ginger; ½ finger-length piece ginger , whizzed to a paste with the garlic; few drops red food colouring (optional)