Slice the small red onion into very thin strands and soak them in ice water for 10 minutes to soften their sharpness and help them curl slightly. Drain the onion very well and pat it dry with a clean towel so no water dilutes the ponzu later. Slice the 3–4 scallions thinly on a slight angle and set them aside. Leave the ginger whole until the last moment, since it is grated fresh just before serving. That thorough draining is what keeps the onion crisp and the dressing sharp instead of watery.

Combine the 80ml soy sauce, 60ml lemon juice, 30ml lime juice, 30ml rice vinegar, 1 tbsp mirin, 1–2 tsp sugar, and the optional 2 tbsp dashi in a small bowl or jug. Stir until the sugar dissolves completely, then taste and adjust: add a little more citrus if it is too salty, a pinch more sugar if it is too sharp, or a splash more soy if it tastes weak. The finished ponzu should be bright, salty, lightly sweet, thin, and pourable, never syrupy. Refrigerate it until serving. That cold, balanced dressing is what supports the beef instead of drowning it.

Remove the 700–800g beef from the fridge 15–20 minutes before cooking so the surface loses its chill and the sear stays fast. Pat it very dry on every side with paper towels, then trim any loose fat or silverskin. Season evenly with 1 tsp kosher salt and ½ tsp black pepper, pressing the seasoning gently onto all sides. Choose a thick, even piece, because a thin steak cannot hold a rare centre through searing. That dry, evenly seasoned surface is what browns on contact instead of steaming in its own moisture.

Heat a heavy skillet or cast-iron pan over high heat until it is very hot and the surface just begins to shimmer, then add the 1 tsp neutral oil. Lay in the beef and sear for about 30–45 seconds on each broad side, then 15–20 seconds on each edge and end, turning it with tongs. The outside should brown well while the centre stays clearly rare and cool to the touch. Remove it immediately, working in a pan large enough that the heat never drops. That brief, fierce contact is what builds a dark rim without cooking the interior.

Transfer the seared beef to a rack or plate and let it cool for a few minutes at room temperature. Then place it in the fridge, uncovered or loosely covered, for 20–30 minutes, until it feels firmed and cold through the surface. Do not wrap it tightly, since trapped steam softens the crust. Because the centre is served rare, keep the beef cold and handle it with clean hands and a clean board throughout. That chilling stage is what halts carryover cooking and preserves the vivid rosy centre.

Set the chilled beef on a clean board and slice it thinly across the grain with a very sharp knife, aiming for neat slices about 3–5 mm thick. Use long, smooth strokes rather than sawing, and wipe the blade between cuts if it drags or picks up juices. Keep the slices in order as you cut so they can be fanned out neatly. Discard any end that is fully grey if you prefer an entirely rosy plate. That clean, cross-grain slicing is what keeps every slice tender and the presentation precise.

Fan the sliced beef in overlapping layers across 4 appetizer plates. Spoon or drizzle the chilled ponzu around and lightly over the beef, so it pools softly without leaving the slices sitting in liquid. Grate the 2–3 tsp fresh ginger and set it in small mounds, then scatter the scallions and 2 tsp toasted sesame seeds lightly and group the drained red onion strands to one side, adding the optional shiso, chives, or daikon cress if using. Serve right away as a cool appetizer. That restrained plating is what lets the beef stay the centrepiece.