Coat the flank steak with the seasoning mix. Place in the bottom of the slow-cooker. Add sliced onions and peppers on top of the seasoned steak. Add lime juice, soy sauce and garlic powder. Cover and cook on high for 5 to 6 hours or until the flank steak is easily shred with two forks. Remove the steak. Shred completely and place back in the slow-cooker. Mix well with the veggies. Drain and serve on soft corn or flour tortillas. Consider toppings such as sour cream, shredded cheese, jalapeño peppers, pickled onions and restaurant-style salsa.