Preheat oven to 325 degrees.
Melt butter in large Dutch oven set to low heat and add sliced onions. Slowly caramelize onions until sticky and brown.
While onions are caramelizing sear each side of chuck roast in a large skillet set to medium-high heat. Approximately 2 minutes per side, or until a brown crust forms.
Deglaze searing skillet with ¼ cup of beef broth and whisk to loosen any burnt on bits from the bottom of the pan.
When onions have sufficiently caramelized pour deglazing liquid into Dutch oven along with an additional ¼ cup of beef broth and increase the heat to deglaze Dutch oven, scraping the bottom of the pot to incorporate burnt bits into the liquid.
Add remaining beef broth and chuck roast to the Dutch oven along with Worcestershire sauce, and mustard. Sprinkle seasonings over the chuck roast. Add BBQ sauce and cover with a lid.
Cover with a lid and place Dutch oven in preheated oven for 3 hours, flipping the roast halfway through cooking.
Roast is done when it is easily shredded with a fork.
Serve the roast whole or shred and serve as sandwiches on buns.