Combine the marinade ingredients in a large bowl, rub into the flank steak and marinade in fridge for 2 hours. Take the steak out of the fridge 20 minutes prior to cooking
Add olive oil to a cast-iron griddle pan on medium-high heat. Cook the steak for 4-5 minutes per side, depending on the thickness of the cut.
Allow the steak to rest on a cooling rack for 10 minutes
In the meantime, chop the ingredients for the pico de gallo and combine in a small bowl
Smash the avocado with a fork in a separate bowl, and mix the cumin seeds and lime juice in. Season with salt and freshly ground pepper
Slice the steak into thin slices- make sure to cut against the grain
Warm up the tortillas on a warm pan, and the tacos are ready to be assembled!