Mix together the yogurt, garlic, ginger, ground coriander and curry powder. Season well. Place the chicken in a large, non-metallic bowl and pour over the yogurt mixture. Toss to mix well, cover and marinate in the refrigerator for 8–10 hours.
Place the spinach in a saucepan and cook over a medium heat for 6–8 minutes. Season and drain thoroughly. Place the cooked spinach in a food processor and blend until smooth.
Heat the oil in a large nonstick frying pan and add the onion. Cook over a gentle heat for 10–12 minutes, then add the cumin seeds and stir-fry for 1 minute. Increase the heat to high, add the chicken mixture and stir-fry for 6–8 minutes. Pour in the measured water and the spinach and bring to the boil.
Reduce the heat to low, cover tightly and cook for 25–30 minutes or until the chicken is cooked through.
Uncover the pan, check the seasoning and cook over a high heat for 3–4 minutes, stirring constantly. Remove from the heat and stir in the lemon juice. Serve immediately.