Heat the butter in a large pan and fry the onion over a medium heat for 5 mins until soft. Add the chicken and continue to fry until turning golden brown. Add the garam masala, garlic, ginger and passata. Leave to simmer, uncovered, for 20 mins.
Just before serving, stir in the cashew nut butter and cream, season to taste and serve scattered with coriander, and with the cooked rice and wilted spinach on the side.