Trim fat from steak. Sprinkle the steak with ½ teaspoon salt and ¼ teaspoon pepper. For a charcoal or gas grill, place the steak on the grill rack directly over medium heat. Cover and grill for 17 to 21 minutes for medium (160°F), turning once halfway through grilling. Remove from the grill; let stand for 5 minutes. Thinly slice the steak diagonally across the grain.
Meanwhile, combine mashed avocados, lime juice, Fiesta Lime seasoning, cumin, and the remaining ¼ teaspoon each salt and pepper in a medium bowl.
Spread Whole-Wheat Individual Pizza Crusts with avocado mixture; sprinkle with cheese. Top with the steak slices, black beans, and tomatoes. Place the pizzas on the grill rack directly over low heat. Cover and grill for 2 to 3 minutes or until heated through. Remove from the grill. Top with sour cream (if desired) and cilantro.