In a small bowl, whisk together soy sauce, lemon juice, lime juice, rice vinegar, mirin, dashi, and sugar until the sugar dissolves. Taste and adjust acidity or salt: add up to 1 tsp more lemon or lime if you want brighter acidity, or 1 tsp more soy for saltiness. Chill in the refrigerator while you prepare the tuna (at least 10 minutes).

Prepare Tuna

Season

Pat the tuna dry with paper towels. Lightly brush all sides with the neutral oil. Season evenly with fine sea salt and freshly ground black pepper.

Sear (Tataki)

Heat a heavy skillet or cast-iron pan over high heat until smoking. Sear the tuna for about 15–20 seconds per side just to create a thin browned crust, rotating to sear all sides (use tongs to hold edges). The center should remain raw. Total sear time should be about 45–60 seconds depending on thickness.

Ice Bath (optional)

If you prefer a very cold center and clean slice, briefly transfer the seared tuna to an ice bath for 30–60 seconds, then pat dry. Otherwise, let rest on a wire rack for 2 minutes to allow residual heat to settle.

Slice

Using a very sharp long slicing knife, trim any uneven ends and slice the tuna across the grain into 1/8"–1/4" (3–6 mm) thin slices. Arrange slices on a chilled plate or platter fanned out or in a single layer.

Assembly & Garnish

Scatter the shredded daikon or microgreens where you will plate the tuna or place a small mound in the center and lay slices over it.

Drizzle 1–2 teaspoons of the chilled ponzu sauce over each serving of sliced tuna, or serve the remainder in a small ramekin for dipping. Avoid over-saturating so the tuna remains delicate.

Sprinkle toasted sesame seeds and sliced green onion evenly over the tuna. Add thin jalapeño slices if using for heat. Place a small dab (about 1/2 tsp) of wasabi on the plate if desired.

Serve immediately as a cold appetizer. Provide chopsticks or small forks and extra ponzu on the side.