01 - Pat the brisket dry and season all sides evenly with salt, pepper, smoked paprika, thyme, and cumin.
02 - Heat olive oil in a large skillet over medium-high heat. Sear the brisket for 3 to 4 minutes per side until deeply browned. Transfer to a plate.
03 - In the same skillet, sauté sliced onions for 5 to 7 minutes, stirring occasionally, until softened and caramelized. Add minced garlic and cook for 1 more minute.
04 - Place half the onions in the bottom of the slow cooker. Set the seared brisket on top, then cover with the remaining onions.
05 - Pour beef broth, red wine if using, and Worcestershire sauce evenly over the brisket and onions.
06 - Cover and cook on low for 6 to 8 hours until the brisket is tender and can be easily pierced with a fork.
07 - Transfer the brisket to a cutting board and let rest for 10 minutes. Slice against the grain into serving portions.
08 - Remove excess fat from the cooking liquid. Serve brisket slices topped with the caramelized onions and spoonfuls of the pan juices.