Keep the hamachi very cold. Using a sharp sashimi or chef's knife, trim any sinew and skin. Slice the fillet into 1/4-inch (about 6 mm) thick slices against the grain to yield 8–10 pieces. Arrange slices on a chilled plate in a single layer.

Make quick marinade

In a small bowl whisk together light soy sauce, mirin, rice vinegar, sesame oil, grated ginger, kosher salt, and black pepper until salt dissolves. Taste and adjust: it should be lightly savory with a hint of sweet and acid. Reserve 1 teaspoon of this mixture for finishing sauce.

Brush or spoon about 1/2 teaspoon of the marinade over each hamachi slice, coating lightly. Let sit at refrigerator temperature for 5–7 minutes to allow flavor to penetrate without 'cooking' the fish.

Citrus-Scallion Garnish & Sauce

Prepare citrus-scallion dressing

In a small bowl combine the reserved 1 teaspoon marinade with the yuzu (or lemon) juice and neutral oil; whisk to emulsify. Stir in sliced scallions, toasted sesame seeds, and thinly sliced jalapeño if using.

Taste and adjust seasoning — add a tiny pinch of salt or a drop more yuzu/lemon if you want brighter acidity.

Crisp Elements & Finish

Optional quick blister or sear

If you want a contrast in texture, heat 2 tablespoons of neutral oil in a small skillet over medium-high heat until shimmering. Briefly (5–8 seconds) sear the surface of each hamachi slice in a single batch to create a faint blister, or spoon hot oil over the slices (aburi technique). Immediately transfer back to the chilled plate.

Arrange cucumber ribbons or daikon matchsticks as a bed on each serving plate. Place hamachi slices over the bed in an overlapping fan or straight line. Spoon the citrus-scallion dressing sparingly over the fish (about 1 teaspoon per portion). Scatter microgreens or shiso leaves, toasted nori strips or crispy shallots, and finish with a light pinch of fleur de sel and an extra dusting of sesame seeds.

Serve immediately as an appetizer or small plate. Provide extra soy/mirin mixture on the side if desired.