In a small bowl whisk tamarind with water. Heat a heavy skillet over medium heat. Add cumin, mustard and coriander seeds; toast, stirring, until they begin to pop; transfer to a coffee or spice grinder along with curry leaves, cinnamon and peppercorns. Process to a fine powder and add to tamarind. In a blender combine garlic, chilies to taste and ginger and process to a paste. Add 2 tablespoons of the mustard oil and spiced tamarind mixture; process until combined. In a large bowl place pork and pour blender contents over; toss to combine. Marinate covered 2 hours at room temperature or refrigerate overnight.
In a large skillet heat remaining 1/2 cup mustard oil over medium-high heat. Add onion and saute 10 minutes, or until lightly browned. Add turmeric, fenugreek, chili powder to taste, coriander and salt; cook, stirring constantly, 30 seconds until spices are fragrant. Reduce heat to medium and add pork and marinade. Cook, turning pork often, 5 minutes. Stir in vinegar and water. Reduce heat to low, cover and cook, about 1 1/2 hours, or until pork is very tender. Serve with rice, garnished with browned onions and cilantro.
*Available in Indian and specialty food store