Heat the mustard oil in a heavy iron karahi over medium heat for about 2 minutes. An iron karahi helps build that deep, rustic Peshawari flavor.
Add the chicken and cook on medium to high heat for 5 minutes, stirring occasionally so it sears lightly without releasing too much water.
Add the whole tomatoes. Cover with a lid and cook for about 10 minutes. Remove the lid and stir occasionally, but avoid over stirring so the tomatoes soften naturally.
Once the tomato skins begin separating, carefully remove the loose skins using tongs. This keeps the gravy smooth and authentic. Also, remove any excess oil, leaving only enough to keep the gravy rich and balanced.
Add minced garlic, salt, and black pepper, green chilies. Reduce the heat to low to medium and gently mash the softened tomatoes using an iron spoon right in the karahi. That light “tik tik” sound is normal and part of the traditional cooking process.
Continue cooking until the oil begins to separate and the aroma of mustard oil and tomatoes becomes rich and intense. Taste and adjust salt if needed.
Finish with fresh julienned ginger and chopped cilantro (or basil if you prefer a milder herb) just before serving.