Preheat oven to 300°F. Position a rack in the center.
Stir salt, pepper, paprika, garlic powder, onion powder, brown sugar, thyme, and chili powder.
Pat brisket dry. Rub the spice mix all over the meat.
Heat olive oil in a large Dutch oven over medium-high heat.
Sear brisket on both sides until deeply browned, 3–4 minutes per side.
Transfer brisket to a plate. Add onions to the pot and cook until softened.
Stir in tomato paste. Cook one minute to caramelize slightly.
Pour in broth, Worcestershire, and vinegar. Scrape up browned bits.
Nestle brisket fat-side up in the pot. Cover tightly with lid.
Braise in the oven until fork-tender, about 3.5–4 hours, basting occasionally.
Optional: Uncover, brush with barbecue sauce, and broil 2–3 minutes for caramelization.
Rest 15 minutes. Slice against the grain and spoon pan juices over.