Heat oil in a large pot over medium heat.
Add cumin seeds and let them sizzle for a few seconds.
Sauté chopped onions until golden brown.
Add minced garlic and grated ginger; cook for 2 minutes.
Stir in the pureed tomatoes and green chilies; cook until oil separates.
Add coriander powder, turmeric powder, and salt; mix well.
Add mutton pieces, stirring to coat them in the spice mix.
Pour in the yogurt, stirring to combine thoroughly.
Cover and cook on low heat for about 45 minutes, or until mutton is tender.
Sprinkle garam masala and garnish with fresh coriander before serving.