Cut lamb into 3/4 " cubes. Place lamb and the bones in a non-metallic bowl with the four tablespoons of oil, the vinegar, tamarind pulp, and salt. Let it marinate at room temperature for eight hours or, refrigerated, for 24 hours. Put two tablespoons of oil, the onion, garlic, and ginger in an electric blender or food processor and run the machine until a fine pasty puree is formed. Heat the one-half cup of vegetable oil in a large enamel coated skillet over medium-high heat. Add the onions and saute until they are caramel brown, stirring constantly to prevent burning. Add the puree. Reduce the heat and add ground cumin, ground mustard, turmeric, red pepper, and paprika. When the spices begin to sizzle and turn dark, in about 15 seconds, add the lamb and bones. Cook until slightly seared (about ten minutes). Add the water and bring to a boil, then lower the heat and simmer, partially covered, until the meat is very tender (about thirty minutes). Pick out and discard bones. Serve over rice.