1Spray 5-quart slow cooker with cooking spray; add onion slices.
2In 10-inch nonstick skillet, heat oil over high heat. Sprinkle taco seasoning mix over pork shoulder, pressing to coat. Brown pork in oil on all sides; remove from skillet to slow cooker. Pour enchilada sauce on top of pork.
3Cover; cook on High heat setting 4 to 5 hours or until tender. Remove pork from slow cooker; shred meat with fork. Place shredded pork in large bowl. Add chiles, 2 cups of the sauce from slow cooker and the onions; stir well.
4Spoon pork mixture onto tortillas. Add toppings. Roll up tortillas.