Add all marinade ingredients to a ziplock bag or big container. Add the flank steak and massage it around a bit. Let it sit in the marinade for at least 4 hours up to 24 hours.
Make the pico de gallo by dicing all the ingredients and adding them to a medium sized bowl. Squeeze the lime over top and then sprinkle on the salt. Mix well and keep it covered and in the fridge until you are ready to use it.
When your ready to make the tacos start by barbecuing the steak.
Heat your bbq to medium-high heat. Around 400 degrees.
Pull your steak out of the marinade and dry it off with a paper towel.
Place on the bbq and cook your flank steak 7 - 8 mins on each side for medium doneness. Remove from the heat, cover and let the steak rest for 5-10 mins.
Cut the steak into thin strips against the natural grain of the steak.
Assemble the tacos by placing a little cheese on the tortilla, then steak, then a little more cheese (if you like) and finally a little pico de gallo and avocado if you wish.