1. Place pork on a cutting board; season with salt and pepper. Heat oil in a large stainless skillet over high heat. Brown pork on all sides, about 6 minutes total. Remove from heat.
2. Place onion in bottom of slow cooker. Whisk in enchilada sauce, 1/2 cup water, lime juice and chili powder. Add pork to slow cooker; cover and cook on HIGH for 6 hours or LOW for 8 hours.
3. Remove pork to a clean cutting board. Cool slightly. When cool enough to handle, separate and discard bones, fat and connective tissue from meat, leaving pork in pieces as large as possible. Place pork on a platter and drizzle with about 1 cup of the liquid from slow cooker.
4. Assemble sandwiches: Spread bottom cut side of rolls with an equal amount of bean spread (about 2 tbsp each), and remaining halves with guacamole. Top bean spread with pork, tomato slices, goat cheese and cilantro.