Pat the brisket dry with paper towels. Mix the salt, pepper, paprika, garlic powder, and mustard powder in a small bowl, then rub all over the brisket. Let it sit at room temperature for 30 minutes.
Heat a large skillet or Dutch oven over medium-high heat. Sear the brisket on both sides until browned, about 3 minutes per side.
Transfer the brisket to a roasting pan or slow cooker. Add the sliced onions, beef broth, Worcestershire sauce, and brown sugar.
For Oven-Baked Brisket: Cover tightly with foil and bake at 300°F (150°C) for 5-6 hours.
For Slow-Cooked Brisket: Cook on low for 8-10 hours or high for 4-5 hours.
For Smoked Brisket: Smoke at 225°F (107°C) for about 10-12 hours, spritzing with apple cider vinegar every hour.
Once cooked, remove from heat and let it rest for 60 minutes before slicing. Always slice against the grain for maximum tenderness.
Notes
For extra tenderness, wrap the brisket in foil during the last 2 hours of cooking. Letting it rest before slicing ensures maximum juiciness!