Heat the vegetable oil in a large pot over medium heat. Add the mustard seeds and let them pop for 30 seconds.
Add the sliced onions and cook for 6 to 8 minutes until they turn golden at the edges.
Stir in the minced garlic and grated ginger. Cook for 1 minute until fragrant.
Add the cumin, turmeric, and Kashmiri chili powder. Stir for 30 seconds to toast the spices.
Add the lamb chunks and salt. Brown the meat on all sides for about 5 minutes, turning pieces as needed.
Pour in the white vinegar and water. Bring the mixture to a gentle boil, then reduce heat to low.
Cover and simmer for 60 to 75 minutes, stirring occasionally, until the lamb is fork-tender and the gravy has thickened.
Remove from heat and sprinkle with chopped cilantro before serving.