Season both sides of the steak well with salt and pepper. In a bowl mix together the remaining ingredients (except for the taco shells) and submerge the steak. Cover with plastic wrap.
Marinate the steak overnight or for at least 30 minutes. (The longer you marinate the more flavor will be infused into the meat) (Remove the steak from the refrigerator about 30 minutes before cooking)
Preheat the grill on medium heat.
Cook steak to desired doneness (see below) using a thermometer.
Let the steak rest for 3 - 5 minutes before slicing against the grain.
Grill the soft taco shells (if using) on each side for 20 - 30 seconds. Serve with the sliced steak.