**Prepare the Brisket:** Pat the brisket dry with paper towels and season with salt and pepper on both sides.
**Optional Sear:** Heat olive oil in a large skillet over medium-high heat. Sear the brisket for 2–3 minutes per side until browned. This step adds flavor but can be skipped.
**Layer the Crockpot:** Place the sliced onions and minced garlic in the bottom of the crockpot. Lay the brisket on top.
**Make the Sauce:** In a mixing bowl, whisk together beef broth, barbecue sauce, soy sauce, Worcestershire sauce, brown sugar, tomato paste, smoked paprika, and ground mustard. Pour the mixture over the brisket.
**Cook:** Cover and cook on LOW for 8–10 hours or HIGH for 4–5 hours, until the brisket is tender and easily pulls apart with a fork.
**Rest and Slice:** Remove the brisket from the crockpot and let it rest for 10 minutes. Slice against the grain for the most tender bites.
**Serve:** Drizzle some of the cooking juices over the sliced brisket and garnish with fresh parsley before serving.