Sprinkle pork roast with salt and pepper. In 3 1/2- to 4-quart slow cooker, place roast and onion.
Cover; cook on Low heat setting 8 to 9 hours.
About 40 minutes before serving, with slotted spoon, remove pork roast and onion from slow cooker; place on cutting board. Discard liquid in slow cooker. With 2 forks, shred pork; return pork and onion to slow cooker.
In small bowl, stir together enchilada sauce and taco seasoning mix. Stir mixture into pork and onion in slow cooker.
Increase heat setting to High; cover and cook 30 minutes longer or until thoroughly heated. Spoon mixture evenly into buns.