Make a recipe of the Fajita marinade, recipe below.
Put steak in a vacuum pouch and add marinade. Chill for an hour before sealing. This marinates the steak AND makes sealing easier as marinade is cold and won't bubble up. Seal and put in water bath at 54.4C (130F) for at least 24 hours.
If adding mushrooms, slice thick and saute them shortly before service in some olive oil or grape seed oil.
If using Big Green Egg, preheat egg to around 500 with cast iron grill. Otherwise heat cast iron pan to med high. When pre-heated, remove steaks from sous-vide pouch reserving marinade liquid. Dry steaks on both sides, and then grill for about 2 minutes per side, or until steaks have some color. You're not trying to cook them as the water bath has taken care of that already. Remove steaks to a plate and allow to rest for 5 minutes.
While steak rests put the reserved marinade into a pan (can use the mushroom pan if you sauted mushrooms) and reduce the marinate to sauce consistency. If there is any liqud forming around the steaks, add that liquid to the sauce.
I like to salt the steaks and let them sit on a plate in the fridge, uncovered, for at least 24 hours before cooking. This isn't mandatory, but adds a little something to the taste.
I like to make the Spanish rice recipe with this.... but whatever starch you're using, if any, put on the plate and then top with the steak sliced against the grain.
Adjust seasonings to the sauce. If you sauted mushrooms, place them on top of the steak, and then top with the reduced sauce.