Preheat the oven to 300 degrees. Lay sliced onions across the bottom of a 9 x 13 metal baking pan.
Pat beef dry and place on top of onions.
In a medium bowl whisk your beef broth, BBQ sauce and apple cider vinegar. Pour over top of beef and give beef a turn so it's coated on all sides.
Place foil on top and crimp around the edges. Make sure to seal tightly so it will stay inside while cooking.
Bake for 2 ½ - 3 hours or until beef is fork tender. If not fall apart, continue to cover and cook until done.
Uncover foil carefully and scoop juices from the pan.
Increase temp to 375 degrees and allow to cook for another 20-25 minutes basting beef with the sauce in between.
Allow beef to rest in the pan for 15 minutes.