Preheat the oven to 500 degrees F. Be sure to let it reach its temperature before roasting the beef. Rub all sides of the beef with the vegetable oil, then rub in the soy sauce, if using, and season with the pepper. Place on a rack over a roasting pan and sear in the hot oven for 15 minutes.
The internal temperature, in the very center, should be 115 degrees F. Immediately transfer the roasting rack to a tray (to catch the juices; do not keep over the roasting pan or the beef will continue to cook).
Set in a cool place to stop the cooking as quickly as possible. In a heavy-duty resealable plastic bag large enough to hold the beef, combine the soy sauce, mirin, green onions, garlic, and lemon zest. As soon as the beef is cool enough to handle, transfer to the bag and refrigerate for at least 6 hours and up to 24 hours, turning over occasionally.
PONZU SAUCE: In a bowl, whisk together the soy sauce, vinegar, sugar, lemon and lime juices, and ginger juice. Keep whisking until the sugar dissolves.
About 45 minutes before you plan to serve, place the bag in the freezer (this will firm the beef and make it easier to cut even slices). After 20 minutes, remove the beef from the bag and discard the marinade.
Slice crosswise with a very sharp knife into 1/4-inch slices, keeping them to an even thickness if possible. Fan the slices, overlapping, on a platter and let stand for 10 minutes. Drizzle with the Ponzu sauce and serve.