Sprinkle the steak with steak seasoning on both sides. Heat olive oil in a frying pan over medium heat until the oil shimmers and place the steak into the hot oil. Quickly brown the steak on both sides, about 5 minutes per side. Set the steak aside.
Stir dark brown soft sugar, onion, red pepper, apple cider vinegar, Worcestershire sauce, passata and tomato puree together in a slow cooker until the dark brown soft sugar dissolves. Place the steak into the slow cooker, immersing it in the sauce. Set the cooker on on High and cook 4 hours or set on Low and cook 8 hours.
To serve, shred the steak with 2 forks or remove the steak and chop and return to the sauce. Toast burger buns or baps and serve the steak on the toasted buns.