In a small bowl, mix together the onion powder, garlic powder, cayenne, cumin, and sea salt. Rub the spices on all sides of the flank steak. Place the flank steak in a 4- to 6-quart slow cooker.
Add the bell peppers, jalapeño, and onion around the steak. Cover and cook on LOW for 8 hours.
Transfer the steak to a cutting board, and with 2 forks or a shredder, shred the meat.
To serve, spoon the shredded meat and peppers on plates and top with avocado, red onion, cilantro, and lime wedges.