Season wagyu generously with kosher salt and black pepper on all sides. Let sit at room temperature for 15 minutes.
Preheat your oven to 500°F. Place your SteakStone or cast iron in the oven and let it heat for at least 40 minutes.
Wash and dry the mizuna or arugula. Set aside.
Once the stone is scorching hot, carefully remove it from the oven and set it on a heat-safe tray or base.
Place the steak directly on the stone and cook for about 2 minutes on the first side. Flip and cook for another 30–45 seconds on the second side.
Remove from the heat and let the steak rest for 4–6 minutes.
Slice the steak thinly against the grain at a bias. You should get 8–10 clean slices.
Arrange the steak slices on a large plate. Add mizuna greens to one corner of the plate.
Drizzle cilantro chimichurri over the steak and greens. Finish with a sprinkle of Maldon salt and a few drops of fresh lemon juice. Serve immediately.