Sprinkle the beef flank steak with steak seasoning on both sides. Heat olive oil in a skillet over medium heat until the oil shimmers, and place the flank steak into the hot oil. Quickly brown the steak on both sides, about 5 minutes per side. Set the beef aside.

Stir brown sugar, onion, red bell pepper, apple cider vinegar, Worcestershire sauce, tomato sauce, and tomato paste together in a slow cooker until the brown sugar dissolves. Place the flank steak into the slow cooker, immersing it in the sauce. Set the cooker on High and cook 4 hours, or set on Low and cook 8 hours.