In a bowl, whisk together the yoghurt, minced onions, minced ginger, garlic paste, red chilli powder, sugar, and salt. Marinate the chicken pieces in this mixture for 15 minutes.
Meanwhile, heat half of the ghee in a large, heavy-bottomed pan over medium heat. Fry the sliced onions and sliced ginger until light golden.
Add the marinated chicken and its marinade. Increase heat to medium-high, bring to a gentle boil, then reduce to medium–low, cover, and simmer for 20 minutes, stirring occasionally.
Stir in the remaining ghee and ½ cup water. Continue to cook uncovered on low heat until the ghee separates from the gravy (about 8–10 minutes).
Add the whole green and red chillies, stir once, then remove from heat when the ghee rises to the surface.