Step 1: Pat the beef brisket dry with paper towels and season generously all over with salt and black pepper.
Step 2: Heat olive oil in a large skillet or Dutch oven over medium-high heat. Sear the brisket for 3-5 minutes per side until deeply browned and a crust forms. Remove brisket and set aside.
Step 3: In the same skillet, add the sliced onion and cook until softened, about 5-7 minutes. Add minced garlic, chili powder, cumin, smoked paprika, and cayenne pepper (if using). Cook for 1 minute until fragrant.
Step 4: Stir in the tomato paste, brown sugar, and apple cider vinegar. Cook for another 2 minutes, scraping up any browned bits from the bottom of the pan.
Step 5: Pour in the beef broth and bring the mixture to a simmer, stirring well to combine all ingredients into a savory sauce.
Step 6: Transfer the seared brisket to your slow cooker. Pour the entire chili-onion sauce mixture over the brisket, ensuring it is well coated.
Step 7: Cover and cook on LOW for 6-8 hours, or on HIGH for 3-4 hours, until the brisket is fall-apart tender. The cooking time will vary depending on your slow cooker and the thickness of the meat.
Step 8: Once cooked, carefully remove the brisket from the slow cooker and transfer it to a cutting board. Tent loosely with foil and let it rest for 10-15 minutes.
Step 9: Slice or shred the brisket against the grain. Skim any excess fat from the sauce in the slow cooker, then pour the sauce over the sliced brisket. Garnish with fresh parsley, if desired, and serve warm.