Add all of the ingredients to a jam or jelly jar with a tightly sealing lid. Seal then shake until well combined. Place the steak in a large ziplock bag or casserole dish. Pour the marinade over the steak, then seal or cover and allow to marinate for 2-8 hours.

In the meantime, prepare the pico de gallo. Mix together all ingredients in a medium sized bowl. The pico can be serve fresh, but is best if refrigerated for a few hours to let the flavors meld together.

Preheat a grill to medium high heat. Lightly oil the grates to keep the steak from sticking. Remove the steak from the marinade and place it on the grill. Grill for 7-9 minutes a side, flipping once. This will yield a medium cooked steak. I don't recommend cooking flank steak above medium. Remove the steak from the grill and allow it to rest for five minutes before slicing thinly across the grain.

If desired, toss the tortillas on the grill for a few seconds to warm and char them slightly. To serve, place steak in the tacos then top with cheese, avocado, pico, and a squeeze of fresh lime juice. Eat immediately.