Rub pork all over with 1 tsp. salt. In a spice grinder, grind peppercorns, cumin, cloves, cinnamon, half of dried red chiles, and paprika. Turn into a bowl and mix with 2 tbsp. vinegar, the tamarind, ginger, garlic, and 1 tbsp. oil. Rub all over pork and chill overnight, covered.
Heat 1 tbsp. oil in a large dutch oven over medium heat. Fry onions until browned, 10 minutes. Scoop out and set aside. Add remaining oil and the pork and fry, stirring often, until lightly browned all over, about 10 minutes.
Return onions to pot along with brown sugar, tomatoes, cardamom pods, remaining whole chiles (crack them first to split), 1/2 tsp. salt, 1 tbsp. vinegar, and 1 cup water. Bring to a boil, then simmer, covered, 1 1/2 hours, or until meat is very tender. (If stew is very liquid after 1 hour, continue simmering with the lid off.) Season to taste with salt, vinegar, and cayenne if using. Top with cashews and cilantro.
*Buy whole-fruit tamarind, compressed into a block, at well-stocked grocery stores and Indian markets. To use, soak a lump in twice as much hot water until it's soft, then push through a fine-mesh strainer.
Note: Nutritional analysis is per serving.