Mix together all the ingredients above into a bowl. Place flank steak into a glass dish or a plastic bag and pour marinade over the top. Refrigerate for at least 2-4 hours or overnight for a more tender piece of meat.
Heat grill or pan on stove top.
Sprinkle both sides of meat with kosher salt and grill to desired temperature.
Allow meat to set for a few minutes before slicing.
Serve with chopped onion and cilantro on corn tortillas.