Season the steak on both sides liberally with salt and pepper. Place the steak on a cooling rack with a baking sheet underneath to catch drips. Let the seasoned steak rest for 45 minutes at room temperature.
Preheat the oven to 200°F.Place the steak in the oven and cook for 30-45 minutes or until it's internal temperature registers 15° lower than your desired finished temp. I wanted a finished temperature of 135°F so I pulled my steak out of the oven when it registered 120°F. Heat the canola oil in a large cast iron skillet until shimmering. The pan should be extremely hot. Place the steak in the pan and sear on both sides. This should not take more than 3 minutes.
Add 1-2 medallions of the butter and the thyme to the pan. When the butter melts, tilt the pan towards yourself and spoon the butter over top of the steak. Transfer the steak to a platter and loosely tent with tin foil and let rest for 10 minutes.
While the steak is resting, add the mushrooms to the skillet and saute over medium-high heat until golden and slightly crispy. This is should take the same amount of time it takes to rest the steak. Remove from the heat.
Carve the steak by following the outline of the bone. Slice the meat into thick strips. Top the steak with the remaining butter medallions and the shiitake mushrooms. Serve immediately with your preferred condiments. I like prepared horseradish.