Tandoori roast chicken

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)