Place roast in slow cooker and season with black pepper.
Mix beef broth, balsamic glaze, honey, stone-ground mustard, Worcestershire, garlic, chili powder, dry onion soup mix, and onions in a medium bowl and pour over roast.
Cook on high for 4 hours, or low for 6-8 hours.
Remove roast from slow cooker and place on a large platter or rimmed sheet pan and shred.
Pour remaining juices over meat and serve hot over mashed potatoes.
Notes
Keep leftovers in an airtight container in the refrigerator for 7 days or in the freezer for 3 months.