Dry roast coriander seeds, cumin seeds, and red chilies until fragrant. Cool completely.
Grind roasted spices with garlic, ginger, green chilies, and tomato into a smooth paste.
Heat oil or ghee in a pressure cooker. Fry onions until light golden.
Add mutton and fry until moisture dries and color changes.
Add masala paste, turmeric, and salt. Cook until oil separates.
Add water, cover, and pressure cook (4 whistles on high, then 15–20 minutes on low).
Fry potato cubes lightly until golden.
Add potatoes to cooked mutton and simmer covered for 4–5 minutes.
Add garam masala and coriander. Simmer for 2 minutes.
Serve hot.