In a large bowl, combine the sliced flank steak, bell pepper, onion, olive oil, fajita seasoning, garlic powder, cumin, salt, and pepper. Toss until the steak and vegetables are well coated.
Transfer the mixture to the slow cooker and spread it out evenly. Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the steak is tender.
Once cooked, shred the steak with two forks and mix it with the vegetables in the slow cooker.
Warm the flour tortillas in a skillet or microwave.
Serve the steak fajitas in tortillas with your choice of toppings such as sour cream, guacamole, salsa, shredded cheese, and fresh cilantro.