Preheat your oven to 300°F (150°C) or prepare your slow cooker on low heat.
Heat olive oil in a large skillet over medium-high heat. Sear the beef brisket on all sides until browned, about 3-4 minutes per side.
Transfer the seared brisket to a roasting pan or slow cooker.
In the same skillet, sauté the sliced onion and minced garlic until softened, about 5 minutes.
Add the beef broth, ketchup, brown sugar, apple cider vinegar, Worcestershire sauce, smoked paprika, ground cumin, chili powder, salt, and black pepper to the skillet. Stir to combine and bring to a simmer.
Pour the sauce over the brisket in the roasting pan or slow cooker.
Cover and cook in the oven for 6 hours or in the slow cooker for 8 hours, until the meat is tender and easily shreds with a fork.
Remove the brisket from the cooking liquid and shred using two forks. Skim any excess fat from the cooking liquid.
Return the shredded beef to the cooking liquid and stir to combine. Let it sit for 10-15 minutes to absorb the flavors.
Serve hot or store for later use.