Bring a small pot of water to a boil, add an onion and boil for 5 minutes. Take out the onion and run it under cold tap water to cool it down.
Blend the garlic, ginger, 4 tbsps of plain unflavored yogurt, tomatoes and a few stems of coriander into a smooth paste.
Grind boneless skinless chicken thighs in a food processor to a smooth paste.
Add the grated ginger and garlic, chopped coriander and green chilies, breadcrumbs, 1 tbsp oil, ½ tsp salt and 1 tsp garam masala powder to the chicken mixture. Mix everything well.
Heat 6 tbsp of oil in a pan and add the tomato-yogurt puree. Stir on medium-low heat until it starts to bubble.
Add 1 tsp salt, 1 tsp sugar, 2 tsp shahi garam masala powder, 2 tsp ground coriander and 1 tsp ground cumin.
Add the chicken koftas one by one in a single layer and cook for 5 minutes.
Gently turn the koftas and add 1 cup warm milk (or ¾ cup water + ¼ cup cream/coconut milk).
Cover partially and simmer on low heat for 25 minutes, giving the pan a gentle shake occasionally.
Add whole green chilies, ½ tsp black pepper and ⅛ tsp ground nutmeg. Taste and adjust seasoning if needed.
Garnish with chopped coriander leaves before serving.
