Add the cumin powder, turmeric powder, mustard, red chili powder, vinegar and salt in a cup and mix well.
Heat the oil in a pan and fry the onion until brown. Add the garlic and stir-fry for few seconds. Then add the spice paste. Stir-fry for a minute. Now add the lamb cubes and fry for 5 minutes. Pour the coconut milk and 1 cup of water and bring it to a boil. Cover and lower the heat and cook for an hour. Serve hot.
You can also cook this in a pressure cooker for 20 minutes after adding the coconut milk and water.