Heat the oil in a heavy-bottomed pan over medium heat. Add the sliced onions and fry, stirring occasionally, until golden brown.
Stir in the garlic and ginger pastes and cook for 30 seconds until fragrant.
Sprinkle in the turmeric, cumin, and red chilli powder; stir continuously for 1 minute to toast the spices.
Pour in 200 ml water to dissolve the spices and create a thick paste, scraping any browned bits from the pan.
Add the chicken pieces, tossing to coat them evenly in the spice mixture. Cook, stirring constantly, for 5 minutes.
Cover the chicken with water (about 200–250 ml more), bring to a boil, then reduce heat to low. Cover and simmer for 25–30 minutes, or until the chicken is cooked through and the sauce has thickened.
Check seasoning and add salt or a splash of water if the gravy becomes too thick. Simmer for 2 more minutes.
Transfer to a serving dish and garnish with chopped fresh coriander, if desired.