Cut each chicken thigh into 3 pieces and place in the pot, then add the sliced onion.
In a bowl mix together the garam masala, tumeric, fenugreek, coriander, salt, and cumin.
Add the spice mix, sliced green chillies, garlic, ginger, sugar and tomato paste. Finely crumble the stock cube into the pot and mix everything together well.
Sprinkle the cornflour to the pot and stir well.
Add the water, lemon juice and chopped tomatoes and mix through.
Cook on low for 5 hours or high for 3 hours.
It's good to taste a small piece before you add them into the recipe to gauge the level of heat that they will produce. If your chillies aren't spicy and you prefer more heat, then you can include some of the seeds, or some ground chilli powder.
If you don't have fresh ginger, substitute with ¼ tsp of ground ginger.
Don't chop them too small. Cutting each thigh into 2-3 pieces usually is about the right size.