Blanch spinach in boiling water for 3 minutes and drown in ice water to stop the cooking of the leaves. Put the wrung spinach in a blender and make a paste. Put aside.
Purée onions. Heat some ghee in a pan with cumin seeds. Add the onion purée and stir till the oil starts to separate from the onion. Add salt, turmeric, cumin and coriander powder. Add a big spoon of tomato paste and some garam masala and fry together. Add the spinach paste, mix, and finally add chicken. Add a little water, cover and let cook, stirring occasionally. Add a spoon of sour cream to make the curry rich once the chicken is mostly cooked. Serve with a drizzle of cream!