1Mix 2 pounds lamb cubes with 1/4 cup vinegar and marinate for 30 minutes at room temperature.
2Process 1 coarsely chopped onion, 3 cloves garlic, and a peeled, sliced 2-inch section of ginger to a paste. Fry in 2 tablespoons oil in a large pan over medium-high heat until fragrant.
3Add 1 tablespoon ground coriander, 2 teaspoons paprika, 1 teaspoon cayenne, 1 teaspoon turmeric, 1 teaspoon ground cumin, 1/2 teaspoon ground fenugreek seed, and 1 whole sprig curry leaf. Stir-fry for several minutes. Remove from the pan and set aside.
4Drain the lamb, reserving the marinade. Add 2 tablespoons more oil to the pan. Add the lamb cubes, working in batches if necessary, and brown lightly. Add 1 13.5-ounce can unsweetened coconut milk along with the reserved marinade and bring to a boil. Lower the heat, and simmer 1 hour uncovered, or until the lamb is tender. Season to taste with salt, black pepper, and a sprinkle of curry powder. Discard curry leaf sprig before serving.