1 tablespoon steak seasoning (such as Montreal Steak Seasoning®)
1 tablespoon olive oil
1/4 cup brown sugar
1 small onion, chopped
1 small red bell pepper, chopped
1 tablespoon apple cider vinegar
1 tablespoon Worcestershire sauce
2 cups tomato sauce
2 tablespoons tomato paste
6 hamburger buns, split and toasted
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 on High and cook 4 hours, or set on Low and cook 8 hours.
To serve, shred the meat with 2 forks, or remove the meat, chop, and return to the sauce. Toast buns, and serve the sloppied steak on the toasted buns.