Lightly season all sides of the tuna with kosher salt. Press the freshly cracked coarse black pepper evenly onto the sides and top of the tuna so it forms a coarse crust; use about 2 tablespoons of pepper to coat evenly.

Sear

Heat a heavy skillet (cast iron or stainless steel) over high heat until smoking. Add 1 tablespoon neutral oil and swirl to coat. Place the tuna loin in the hot pan and sear 20–30 seconds per side for rare (aim for a warm, red center). For medium-rare, sear 40–45 seconds per side. Do not move the tuna while searing so a pepper crust forms. Remove from pan and transfer to a cutting board to rest 2 minutes.

Slice

Using a very sharp knife, slice the tuna across the grain into 1/4-inch thick slices. Arrange slices neatly on a chilled serving plate or individual plates for small plates/appetizers.

Ponzu Sauce

While the tuna rests, whisk together 3 tablespoons ponzu, 1 teaspoon light soy sauce, 1 teaspoon mirin, 1/2 tablespoon fresh lime juice, 1/4 teaspoon sesame oil, and 1/4 teaspoon granulated sugar in a small bowl until sugar dissolves. Taste and adjust: add more ponzu for brightness, more lime for acidity, or a pinch more sugar if too sharp. Keep sauce at room temperature.

Option: Warm the sauce slightly (microwave 5–7 seconds) if you prefer a slightly warm drizzle, but serving at room temperature is traditional.

Garnish & Finish

Drizzle & finish: Lightly brush or drizzle a thin film of sesame oil over the sliced tuna (use about 1/4 teaspoon total). Spoon 1 to 1 1/2 tablespoons of the ponzu sauce over each portion of slices, or serve the sauce pooled alongside for dipping.

Garnish: Scatter the finely sliced scallions over the tuna, sprinkle toasted sesame seeds evenly, and add microgreens or baby cilantro if using. Serve with lime wedges on the side for optional extra brightness.

Serve immediately as a small plate/appetizer so the tuna remains tender and the pepper crust stays crisp.