Combine beer, orange juice, orange halves, salt and pepper in a large zip top bag. Add the flank steak and seal. Marinate for at least 1 hour or up to overnight.
Before grilling, remove steak from refrigerator for 30 minutes to remove the chill. This helps it cook more evenly. Meanwhile, preheat grill to medium-high heat. Remove steak from marinade and grill to desired doneness. Place orange halves on the grill for a few minutes to char. Remove steak from grill and allow to rest for 10 minutes while you assemble the toppings. Squeeze (with tongs) the orange halves over the steak. If desired, place tortillas on the grill quickly to toast and slightly char. Set aside.
For the creamy avocado sauce: put all ingredients in a food processor or blender and blend until completely smooth.
Slice steak against the grain and top tortillas with it along with desired garnishes. Drizzle with creamy avocado sauce and enjoy.