Blend the garlic and chilli to a smooth paste in a wet paste grinder or food processor. Put the chicken in a bowl and add half the ginger and garlic paste and the turmeric. Mix well and set aside.
Put a heavy based saucepan over a medium heat and add the oil. When hot add the birds eye chillies and let them sizzle for a few seconds. Add the onions and fry for 9-10 minutes until they soften and begin to change colour.
Add the remaining ginger and garlic paste and fry for a further minute, stirring well to make sure it doesnt stick to the bottom.
Add the chilli powder and ground coriander and stir well for 20 seconds. Add the chicken and increase the heat slightly. Fry the chicken pieces for 3-4 minutes to seal, then season to taste and add the sugar.
Turn the heat to low, cover and let the chicken cook in its juices for 15 minutes, stirring halfway through cooking to make sure it doesnt stick to the bottom of the pan.
Add most of the sliced mango and mix well, then cook for another 7 minutes, stirring halfway. Add the garam masala and 2-3 tbsps water to create a light sauce.
Garnish with the remaining sliced mango and the chopped fresh coriander. Serve warm with chapattis or parathas and some raita.