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To make the onion purée, bring a small pan of water to the boil and add half the roughly chopped onion. Boil until soft (about 10 mins), drain and puree with a hand blender or mini food processor.
Heat a large saucepan on a high heat. Once it's hot add the oil and finely chopped onions and reduce to a low heat. Cook the onions slowly and gently until golden brown in colour, and season.
Add the ginger and garlic purée, curry powder, turmeric, chilli powder and a splash of water to the pan of onions and stir in well. Fry for a couple of mins. Add the diced chicken thighs and stir in well.
Mix the yogurt, tomato purée and onion purée together in a jug with 300ml water. Pour into the saucepan and mix well. Turn up the heat until the sauce begins to boil. Simmer for 15 to 20 mins, stirring occasionally.
Finally, sprinkle in the garam masala and stir in well for the final 2 mins before serving.