1If pork is tied, remove strings or netting. Place pork in 3- to 4-quart slow cooker. Sprinkle with taco seasoning mix. Top with beans.
2Cover and cook on low heat setting 8 to 10 hours.
3Remove pork from cooker; place on cutting board. Shred pork, using 2 forks. Return pork to cooker and mix well. Stir in cheese.
4To serve, cut muffins crosswise in half. Place each bottom half on plate; top with 1/2 cup pork mixture, 1 tablespoon lettuce, 1 tablespoon tomato and muffin top. Serve with sour cream; sprinkle with cilantro.