Heat the vegetable oil in a large pan over medium heat.
Add the cumin seeds and let them sizzle for a few seconds until they start to splutter.
Add the chopped onions and sauté until they become translucent and start to brown, which should take about 5 minutes.
Stir in the minced garlic and ginger and cook for another 2 minutes until fragrant.
Add the chopped tomatoes and green chili pepper to the pan. Cook for 5-7 minutes, or until the tomatoes are soft and have started to break down.
Add the ground coriander, ground cumin, turmeric powder, and chili powder. Stir well and cook for 2-3 minutes to allow the spices to toast and release their flavors.
Add the cubed lamb to the pan and brown it on all sides. This should take about 5-7 minutes.
Pour in the coconut milk and water. Season with salt to taste. Stir everything together, then bring the mixture to a simmer.
Cover the pan with a lid and let the curry simmer for 45 minutes to 1 hour, or until the lamb is tender and the sauce has thickened. You may need to add more water if it gets too dry during cooking.
About 10 minutes before the curry is done, stir in the garam masala.
Once the lamb is tender and the sauce has reached your desired consistency, remove the curry from heat.
Garnish with fresh cilantro leaves.Serve your delicious lamb curry with rice or naan bread. Enjoy!