Trim excess fat from the beef brisket to about ¼ inch. Rub brisket with smoked paprika, ground cumin, black pepper, and salt.

Sauté sliced onions in a large skillet over medium heat for 15-20 minutes until golden brown. Add minced garlic and sauté for an additional 2-3 minutes.

Transfer caramelized onions to slow cooker or Dutch oven, place seasoned brisket on top. Mix brown sugar, soy sauce, beef broth, and apple cider vinegar; pour over brisket.

For slow cooker, cover and cook on low for 8-10 hours. For Dutch oven, preheat oven to 300°F (150°C) and bake covered for 3-4 hours.

Remove brisket from heat and rest for 20 minutes. Use two forks to shred brisket and mix in cooking liquid.