Cut lamb into 3/4" cubes. Place lamb and the bones in a nonmetallic bowl with the oil, vinegar, tamarind and salt. Marinade at room temperature for 8 hours, or refrigerated for at least 24 hours.
Put two tablespoons oil, onion, garlic and ginger in a food processor and puree.
Heat 1/2 cup oil in large skillet over medium-high heat. Add onions and saute until they are caramel brown, stirring constantly to avoid burning. Add the puree. Reduce the heat and add cumin, mustard, turmeric, red pepper, paprika, sumac and cinnamon. When the spices begin to sizzle and turn dark (about 15 seconds), add the lamb and bones; reserve the marinade. Cook until slightly seared (about 10 minutes). Add water to the marinade to make 2 1/2 cups liquid; add to pan and bring to a boil, then lower the heat and simmer, partially covered, until meat is very tender (about 30 minutes).
Pick out and discard the bones and serve over rice.
From the Dinner Co-op