Spray 5-quart slow cooker with cooking spray. Place onion in cooker.
Sprinkle taco seasoning mix over pork shoulder, pressing to coat. Place pork in cooker. Top with enchilada sauce.
Cover; cook on High heat setting 5 to 6 hours or until tender. Remove pork from slow cooker; shred meat with fork, discarding fat. Place shredded pork in large bowl. Strain sauce. Add chiles, salt and 1 cup of the strained sauce. Stir well.
Spoon about 3/4 cup rice into each of 8 individual serving bowls. Top each with about 1/2 cup pork mixture and 1/4 cup lettuce. Garnish with salsa, cheese and/or cilantro.