Coat steak on both sides generously with coarse Kosher salt; use meat cleaver on both sides to break the steak down; place in refrigerator for at least 45 minutes
Remove steak from refrigerator and rinse well; (avoid allowing steak to touch the sink)
Dry steak thoroughly with paper towels and season with salt and pepper on both sides.
Preheat oven to 450° (convention method preferred)
Mean while place 2 tbsp of butter in 10” cast iron skillet and heat on high. Watch closely• When hot, sear both sides of the steak for 2-3 minutes. Sear close to the final color you want. Transfer pan to the preheated oven.
Now comes the variables. The steak thickness, how long you seared, and the true temperature of the oven. It takes about 7-8 minutes to get to about 145°(medium). Remove from the oven and allow to rest for a few minutes before serving. Cook to the proper temperature; not by time.
Remove from oven and top with remaining butter. Remember the steak will continue to cook when removed from oven. Allow to rest for at least 15 minutes.
Top with caramelized onions and mushrooms. Serve immediately.