Marinate the flank steak in Stubb’s Beef Marinade at least 4 hours (preferably overnight).
Prepare the grill for cooking on high direct heat. Grill until preferred doneness (about 5-6 minutes per side for medium rare).
Allow steak to rest for 5 to 10 minutes, covered in tin foil.
Slice it thinly against the grain.
Fill corn tortillas with flank steak, sliced mango, red onion, guacamole, shredded cheese and tomatoes.