Pat the hamachi fillet dry with paper towels. Lightly season both sides with the kosher salt and let sit 2 minutes to firm the flesh. Using a very sharp knife, slice the hamachi into 1/8–1/4 inch (3–6 mm) thin sashimi slices against the grain. Arrange slices on a chilled platter in a single layer, slightly overlapping if desired.

Make the dressing

In a small bowl whisk together the soy sauce, mirin, fresh lime juice, and sesame oil until combined. Taste and adjust balance: add a little more mirin for sweetness, lime for acidity, or soy for saltiness. This will be the light marinade/pour-over for the hamachi.

Immediately before serving, spoon or lightly drizzle about half of the dressing evenly over the arranged hamachi slices so each piece gets a thin coating. Reserve the other half for optional dipping or extra finishing.

Finishing & Garnish

Scatter the thinly sliced scallions and microgreens over the dressed hamachi slices. If using jalapeño, distribute the thin rounds sparingly over the fish for a touch of heat.

Sprinkle the toasted sesame seeds evenly across the fish for texture and aroma. Tuck cucumber ribbons or daikon lightly around the edge of the platter as a refreshing contrast.

Serve immediately with lime wedges on the side to squeeze over the fish. Offer optional condiments — wasabi, pickled ginger, and extra ponzu/soy — so diners can customize each bite.