Heat the oil in a pan and fry the chicken until cooked. Remove from the pan and set aside.
Add the onion slices to the pan and fry until golden brown (but not burnt), add the garlic and ginger paste and fry together for a couple of mins.
Add the cumin seeds, coriander, chilli powder, turmeric, fenugreek and cinnamon and fry together for around 3-4 mins.
Add the chopped tomatoes, then refill the can with water (this will use up any left over tomatoes) and add half the water and stir together.
Pour into a food blender and blitz to a smooth sauce then pour back into the pan. Using the left over water, use to rinse the blender and add this to pan.
Add the chicken and caster sugar, stir together and then gently simmer for around 10mins.
Add the garam masala and greek yoghurt, stir together and simmer for a further 5mins.
Serve immediately along with boiled rice, naan and poppadums.