Heat mustard oil in a large pan until slightly smoking, then reduce heat.
Add onions and fry until soft and light golden, about 6–8 minutes.
Stir in ginger and garlic and cook for 1–2 minutes until fragrant.
Add turmeric, red chilli, coriander powder and salt, cook for 1 minute.
Add chicken and coat with the spice mixture, cook for 5 minutes.
Add tomato and mustard paste, pour in 200 ml water, and bring to a gentle simmer.
Cover and cook for 20 minutes until chicken is tender.
Add green chillies and sugar (if using), simmer uncovered for 2–3 minutes.
Garnish with chopped coriander and serve hot with steamed rice.
