First you need to prep your steaks. Drizzle olive oil on all sides of each steak and gently rub it around to evenly coat the meat. Then sprinkle kosher salt generously on each side. You need to cover the meat with salt. Remember, the thicker the steak, the more salt you need to add.
Now it's time to add the pepper. If you've already made the Steak Pepper Rub, use that, otherwise, you can just use fresh ground black pepper. Rub this in all over each steak. Then place a bakers rack on top of a backing sheet and lay the steaks on the rack. This will give the steaks the ability to dry out just a bit on all sides.
Refrigerate uncovered for at least 2 hours, and up to over night.
Take the steaks out of the fridge about an hour before you're ready to grill. And speaking of Grilling, we're going to be using cast iron on the grill. There's a very good reason for this, and it has to do with the Maillard reaction that causes those awesome grill marks when you grill a steak right on the grates. Those marks are where the best flavor lives on your steak, so we're going to maximize that flavor by extending the marks over the entire steak. Again, trust me on this.
Take the steaks out of the fridge about an hour before you're ready to grill. While the steaks are sitting out, get your grill ready. By ready, I mean hot. Super hot. Super, super, super hot. You need somewhere between 500F and 600F to really make this work. Place 2 12 inch cast iron skillets (or 1 12 inch cast iron skillet and a cast iron Plancha) on the grill over direct heat. Put these on the heat immediately, while the grill is warming up. These things need to be REALLY hot.
When the grill's ready, place 2 steaks in a skillet, and leave them there for 2 1/2 to 3 minutes. Take the steaks off the grill and set them on a cutting board or platter. Cover them with some aluminum foil to keep them warm.
Quickly add the mushrooms, onions, stock, and wine to the skillet (keep the skillet on the fire – I moved it off here for the photo).
Place the butter on top of the mushroom mix and stir well to get everything mixed in.
Throw the steaks down in the second skillet, or on the plancha (on the unseared side, of course) and cook for 3 to 4 minutes.
When the steaks are done, pull them off the grill. Your mushroom mix might be done now, as well, but, if not, leave the skillet on the grill while you plate the rest of your meal.