How to Make It
In a medium bowl, mix together the red onion, 4 tablespoons of the vinegar, and ¼ teaspoon each salt and pepper. Cover and refrigerate for at least 4 hours and up to 3 days.
In a large bowl, whisk together the sour cream, mayonnaise, 2 tablespoons of the remaining vinegar, and ¼ teaspoon each salt and pepper. Fold in the broccoli slaw and refrigerate for up to 12 hours.
In a 4- to 6-quart slow cooker, mix together the yellow onion, tomato paste, sugar, Worcestershire sauce, mustard, Cajun seasoning, the remaining 2 tablespoons of vinegar, ½ teaspoon salt, and ¼ teaspoon pepper. Add the pork and turn to coat.
Cover and cook until the pork is very tender, on low for 7 to 8 hours or on high for 4 to 5 hours (this will shorten total recipe time). Using 2 forks, shred the pork and mix it into the cooking liquid. Serve the pork and onions on the toast with the broccoli slaw.