Place onions in slow cooker. Sprinkle turkey with salt and place in slow cooker, meat side up. Cover and cook on low until internal temperature of turkey registers 165ºF on an instant-read thermometer, 4 to 6 hours.
Remove turkey meat from bone, shred it, then toss with enchilada sauce. Place 1/4 cup turkey inside each taco shell and top with lettuce, avocado, salsa and shredded cheese.