Place steak, butter, steak sauce, garlic, salt, and black pepper in a large resealable plastic bag; shake until marinade is well-mixed. Seal the plastic bag most of the way but keep one small corner open on one end.
Fill a deep baking pan with warm water. Submerge plastic bag in the water slowly, allowing the pressure of the water to push all the air out and mold the bag around the steak. Seal the bag completely and let sit in the water, about 2 hours.
Preheat oven to 235 degrees F (112 degrees C).
Pour water out of the baking pan. Line pan with aluminum foil. Remove steak from the bag and place it on the pan. Pour marinade around steak. Gather sides of the aluminum foil into a tight ball around the steak.
Bake in the preheated oven until an instant-read thermometer inserted into the center reads 130 degrees F (54 degrees C) for medium, about 20 minutes.
Remove steak from oven. Uncover and season both sides generously with salt and black pepper. Allow steak to rest.
Preheat an outdoor grill for high heat. Place a cast iron skillet on the grill and heat to 500 degrees F (260 degrees C).
Cook steak in the hot skillet until it forms a crust and an instant-read thermometer inserted into the center reads 140 degrees F (60 degrees C) for medium, about 1 minute per side.
