Heat the oil in a large heavy-based pan over a medium heat. Add the onion and cook, stirring for 5 to 6 minutes until soft. Add the cumin, coriander, turmeric, cayenne pepper, garlic and ginger, and cook, stirring for 2 minutes more. Add the chicken and increase the heat to medium-high. Cook, stirring often until the chicken is browned, about 5 minutes.
Stir in the tomatoes and salt and bring to simmering point. Reduce the heat to low, cover the pan and simmer gently for 15 minutes. Add the brown sugar, lime juice and baby spinach and stir until the spinach has just wilted. Remove from the heat, sprinkle with the chopped coriander and serve with steamed rice.