Sprinkle the steak evenly with the salt and spices. Put the steak in a small (quart) vacuum pouch, and vacuum seal the bag. Put the bag in the freezer for later. (Up to a year, according to the USDA)
2. Sous Vide the steak
Remove the bag from the freezer, put it in the sous vide machine, and sous vide at 140°F/60°C for at least 1 1/2 hours, up to 6 1/2 hours. Use 130°F/54.5°C for medium-rare. The kids like their steak medium, so that’s how I cooked this one.
3. Set the grill for direct high heat
Set the grill up for cooking on direct high heat, and clean the grill grate. For my gas grill, I preheat with all the burners on high for 15 minutes, then brush the grate clean with my grill brush.
4. Sear the steaks over direct high heat
Remove the steaks from the vacuum bag. Pat the steaks dry with paper towels, then put the steaks on the grill over direct high heat. Sear, flipping every minute or two, until the steaks are well browned on both sides, about 4 minutes total.
5. Rest and serve
Let the steak rest on a plate for ten minutes, then slice thin on the bias and serve with tortillas, salsa, sour cream, and other fajita toppings.
Of course, you don't have to freeze the vacuum sealed bag with the steak - it can go straight into the sous vide cooker if you're in a hurry, or it can be refrigerated for a couple of days if you know you'll be cooking it soon.
Why use the grill? Because it’s the quickest way for me to warm up 20 flour tortillas - I spread them out on the grill over medium heat, about 1 minute a side, and wrap them in a clean kitchen towel to keep them warm. After the tortillas were ready, I upped the heat to high and seared the steak, and threw on some peppers and onions as an additional topping.
Don’t feel like grilling? Get a pan ripping hot, add a teaspoon of oil, and sear the steak quickly, about 1 minute per side.