Pat the tuna loin dry with paper towels. Trim into a clean rectangular block if needed. Season all sides lightly with kosher salt and freshly ground black pepper, pressing gently so it adheres.

Sear tuna

Heat a heavy skillet (cast iron recommended) over high heat until smoking hot. Add 1 tablespoon neutral oil and swirl to coat. Using tongs, sear the tuna for 20–30 seconds per side to develop a dark brown crust, including the edges, rotating to sear all faces. The interior should remain rare (bright red). Total sear time about 1.5–2 minutes depending on thickness.

Cool and slice

Transfer the seared tuna to a cutting board and let rest 1–2 minutes. Using a very sharp knife, slice into 6–8 thin medallions (~1/4 inch thick) on a single stroke, wiping the blade between cuts for clean edges. Arrange slices on a chilled serving plate or wood board, slightly overlapping if desired.

Chef's special sauce

Combine 3 tbs soy sauce, 1 tbs mirin, 1 tsp rice vinegar, 1/2 tsp sesame oil, 1 tsp grated ginger, 1/2 tsp grated garlic, 1/2 tsp sugar, 1 tsp yuzu or lemon juice, and 2 tbs finely chopped scallion in a small bowl. Whisk until sugar dissolves and flavors are integrated. Taste and adjust: add a few drops more yuzu/lemon for brightness or 1/2 tbs water if too salty. Keep at room temperature until plating.

Garnish & serving

Plate and garnish: Spoon or lightly brush a small amount (about 1 tsp) of the chef's special sauce across each tuna slice or make a shallow pool beneath the arranged slices. Scatter thinly sliced dark green scallion on top of the tuna for color and crunch.

Add caviar: Using the back of a small spoon, place a small dot (~1/8–1/4 tsp) of caviar or roe onto 2–3 representative slices (or distribute tiny pearls across all slices for an elegant look).

Finish: Sprinkle toasted sesame seeds over the plate, tuck a few microgreens near the tuna, and place pickled ginger and a small dollop of wasabi on the side. Serve immediately so the tuna remains rare in the center.