Marinate the chicken: In a large bowl, combine chicken with yogurt, 1 tbsp ginger-garlic paste, turmeric, chili powder, 1 tbsp mustard oil, and 1 tsp salt. Mix thoroughly and refrigerate for at least 30 minutes (or up to 4 hours).

Fry the potatoes: Heat 4 tablespoons of mustard oil in a heavy-bottomed kadai until it begins to smoke lightly. Lower the heat. Rub potato halves with a pinch of turmeric and salt, then fry in the oil until golden-brown on all sides. Remove and set aside.

Tempering whole spices: In the same hot mustard oil, add 1 tsp sugar (to promote deep mahogany color), bay leaves, bruised cardamoms, cinnamon stick, and cloves. Let them crackle for 20 seconds.

Caramelize onions: Add the finely sliced onions. Sauté on medium heat patiently for 12-15 minutes until they turn a deep rich reddish-brown (barista stage). Do not rush this step—it defines the classic kosha color!

Add aromatics & tomatoes: Stir in the remaining ginger-garlic paste and sauté for 2 minutes until raw smell dissipates. Add pureed tomatoes, coriander powder, cumin powder, and Kashmiri chili powder. Cook until oil separates from the masala.

Slow-braise chicken (Koshano): Add the marinated chicken and slit green chilies. Stir well on high heat for 5 minutes. Lower the heat to medium-low, cover, and let the chicken slow-cook in its own juices for 20-25 minutes, stirring every 5 minutes so it does not catch at the bottom.

Simmer with potatoes: Add the fried potatoes and ½ to ¾ cup warm water. Cover and simmer on low heat for 12-15 minutes until chicken is fork-tender and potatoes are soft.

Finish & rest: Sprinkle freshly ground Bengali garam masala and a spoonful of pure ghee. Turn off heat, cover, and let it rest for 10 minutes before serving. The gravy will turn glossy, thick, and cling perfectly to the chicken.