Heat the oil in a heavy-based pan. Fry the onions gently for 5 – 10 mins until soft. Add the garlic, ginger, coriander, mustard seeds, garam masala and cumin and cook for 1 - 2 min allowing the aromas to release.
Add the chicken and cook for 10 mins over a gentle heat, flipping occasionally and making sure the spices don’t catch. Pour in around 300 ml boiling water until almost covering. Stir in the stock cube and cinnamon stick. Simmer for around 45 mins - 1 hour with the lid off so there is a small amount of thickened sauce at the bottom of the pan. Remove the cinnamon stick.
Stir in the yogurt and sultanas, heat through gently and serve. Scatter with coriander, if using.