Mix all the marinade ingredients in a food mixer. Lay the steaks in a shallow tray and pour over the marinade. Turn the steaks to coat them in the mix and leave for several hours or over night in the fridge.
Heat the BBQ. When it is hot, wipe any excess marinade from the steaks, then cook for 3 minutes on each side for medium-rare or longer if you prefer a more well-done steak. Allow to rest for 5 minutes and then cut into thick slices.
Warm the tortillas, place the sliced meat over, and serve with the sour cream and add guacamole and any other accompaniments that you fancy!