Prepare and Season: Pat brisket dry. Mix salt, black pepper, paprika, smoked paprika, oregano, onion powder, garlic powder, cumin, and cinnamon in a small bowl. Rub spice mixture all over brisket.
Sear: Heat oil in a large skillet over medium-high heat. Sear brisket for 3-4 minutes per side until browned. Transfer to slow cooker.
Cook Aromatics: In the same skillet, cook onions for 4-5 minutes until softened. Add garlic and cook 1 minute. Stir in tomato paste and cook 1 minute more.
Make Sauce: Add beef broth, apple cider vinegar, and brown sugar to skillet. Bring to a simmer, scraping up any browned bits.
Slow Cook: Pour sauce and onions over brisket. Cover and cook on low for 8 hours or high for 5 hours, until brisket reaches 195-205°F internal temperature.
Rest and Slice: Remove brisket and let rest 10-15 minutes. Slice against the grain.
Serve: Skim fat from sauce. Pour sauce over sliced brisket or serve on the side.