Combine the allulose, olive oil, tamari, Worcestershire sauce, garlic, ginger, and pepper in a mixing bowl and whisk well to combine. Pour into a large food storage bag. Add the steak, push out any air, seal the bag, and flip a few times. Refrigerate for 12-24 hours, flipping the bag occasionally.
Remove the steak from the refrigerator, discard the marinade, and let the steak sit for 30-60 minutes to come to room temperature.
Heat a large cast iron skillet over high heat for 5 minutes. Pat the steak dry and add to the hot skillet. Cook for 2-3 minutes or until nicely browned then, using tongs, flip and cook another 2-3 minutes. Turn off the heat but leave the steak in the pan. Dot the butter on top and cover the pan loosely with a piece of foil. Let sit for 5 minutes or until the steak reaches an internal temperature of between 120-125 degrees for rare, 130-140 for medium-rare. Remove the steak from the pan and let rest for 10 minutes before slicing against the grain.