To the sleeve of a slow cooker add onion, garlic, tomatoes, chipotle peppers, Worcestershire sauce, and cilantro. Add pork and season with salt and pepper. Cover and slow cook on high for 4 to 6 hours until tender.
For the pickled onions:
In a small saucepan over medium heat, add vinegar, sugar, red pepper flakes and salt, heat until sugar dissolves. Remove from heat and add onions and dried oregano. When mixture is cool pour into a quart container. You may need to add a bit of water just to cover. (Keep at room temperature for about 6 hours then place in the refrigerator.)
Remove pork from slow cooker and shred using 2 forks. Toss with 1/2 cup of the liquid from the slow cooker.
To serve, place 1/4 cup of the pork on a tortilla, top with cabbage, avocado, onions, sour cream and cilantro. Repeat with remaining ingredients.