01 - Preheat oven to 300°F.
02 - Combine kosher salt, black pepper, smoked paprika, garlic powder, and onion powder; rub evenly over brisket.
03 - Place sliced onions in the bottom of a roasting pan; set brisket on top, fat side up.
04 - Pour 2 cups of beef broth around the brisket without wetting it; cover pan tightly with foil.
05 - Bake brisket at 300°F for 4 hours, or until very tender.
06 - Combine all sauce ingredients in a saucepan; simmer over medium heat for 15–20 minutes until thickened.
07 - Remove brisket from oven and increase oven temperature to 400°F.
08 - Drain excess liquid from pan, brush brisket generously with barbecue sauce; return uncovered to oven for 20–30 minutes until caramelized.
09 - Let brisket rest for 15 minutes; slice against the grain and serve with additional sauce and onions.