Place onions in crock pot.
Blend brown sugar, paprika, salt and pepper; rub over roast.
Place roast in crock pot. Drizzle liquid smoke over roast.
Combine vinegar, Worcestershire sauce, red pepper flakes, sugar, mustard, garlic and cayenne pepper and stir to mix well.
Drizzle about ⅓ of mixture over roast. Cover and refrigerate remaining mixture.
Cook roast on low for 8-10 hours.
Remove meat and onions, discard onions and shred meat. Add juices from crock pot and remaining vinegar mixture.
Serve on Martin’s Sandwich Potato Rolls with choice of sauce(s).