Set up your grill for smoking at 225° Fahrenheit. Use a cast iron skillet below the main grate as a heat diffuser.
Start by making your binder. This binder will allow the seasoning to stick to the meat. In a small bowl, mix together the Wagyu beef tallow with ½ cup whiskey until smooth. Use your hands to coat the steak with the whiskey-tallow binder.
Season the steak generously with salt and pepper and set aside.
Add about 1/3 cup of the whiskey-tallow binder to the cast iron skillet in the base of the Green Egg. Add the mushrooms and place the grate on top.
Place your whiskey steak on top of the grate, making sure it’s placed in the center above the mushrooms in the cast iron skillet. As it smokes, the fat will render and drip into the mushrooms. Place the temperature probe in the thickest part of the steak, close the lid and smoke at 225°F, and wait for the internal temperature to come up to about 115°F. This will take 45 minutes to 1 hour.
Once the steak has reached 115°F, take it off the grill and wrap it loosely with aluminum foil to allow it to rest.
Now it’s time to set up the smoker for searing. Open up all smoker vents all the way to get the air moving over the charcoal. Remove the grate so you can work in the cast iron skillet. You're aiming for about 500° Fahrenheit in the skillet.
Move the mushrooms to the side, and place a measuring cup with 1/3 cup of whiskey in the pan near the edge to warm the whiskey.
Add the remaining whiskey-tallow binder to the hot cast iron skillet. Add your steak to the skillet and sear for 30 seconds on each side.
Pour the whiskey that's been warming in your measuring cup over and around the steak and light it with a lighter or torch to pan flambé the steak. Flambé until the flames extinguish on their own, turning the steak once. This should take about 1 minute.
Now it’s time to set this steak on fire (again!) for one final sear. Use your Su-V Gun flamethrower from GrillBlazer (see link under equipment for a discount) to get to the perfect color. If you don't have a flamethrower, just keep searing in the cast iron skillet, turning every 30 seconds until the color is a deep brown/red.