Season the pork butt all over with 1 teaspoon salt and 1/4 teaspoon pepper. Lay the onion rounds in one layer in the bottom of the insert of a slow cooker. Lay the pork butt and the chorizo on top of the onions. Whisk together the ketchup, 1/4 cup of the vinegar, Worcestershire, molasses, chipotles, soy sauce, chili powder, dry mustard and garlic in a medium bowl; pour over the pork butt. Set the slow cooker on high for 8 hours, or until the pork is fork-tender.
Carefully transfer the pork butt and chorizo to a rimmed baking sheet. Discard the onions, and skim any excess fat from the cooking liquid. Transfer the liquid to a medium saucepan, and simmer over medium-high heat until reduced to about 1 1/2 cups, 8 to 10 minutes. Add the reduced sauce back to the slow cooker.
Meanwhile, shred the pork with two forks. Finely chop the chorizo. Return the pork and chorizo to the slow cooker. Stir in the remaining 2 tablespoons vinegar. Check for seasoning, and add more salt if necessary. Serve on rolls with coleslaw.
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