Keep the yellowtail well chilled until ready to slice. Pat the fish dry with paper towels to remove excess moisture; this improves knife control and presentation.

Trim and portion

Using a very sharp fillet or sashimi knife, trim any bloodline or dark tissue from the yellowtail. Portion the 10 oz loin into 10–12 thin slices (about 1/8"–1/4" thick), cutting on a slight diagonal across the grain to get tender, wide slices suitable for sashimi.

Prepare jalapeño

Wash the jalapeño and slice it into very thin rounds (use a mandoline or sharp knife). Remove seeds for milder heat, or leave some seeds for spice. You should have about 12 thin rounds to top each slice of fish.

Finish garnishes

Lightly toast sesame seeds in a dry skillet over medium heat, shaking until fragrant (about 1–2 minutes). Pluck cilantro leaves and, if using scallion, slice very thinly on the bias. Keep garnishes ready for plating.

Season fish lightly

Arrange the yellowtail slices on a chilled serving plate in a single layer or slightly overlapping. Sprinkle the kosher salt evenly over the slices (use about 1/4–1/2 tsp total), and drizzle the neutral oil in a thin stream over the fish to give a glossy finish (approx. 1 tsp total).

Top and finish

Place a jalapeño round on each slice (or every other slice, according to preference). Scatter cilantro leaves and microgreens or scallion over the fish, and finish with a light sprinkle of toasted sesame seeds.

Ponzu Sauce

Combine ingredients: In a small bowl, whisk together the lemon juice, lime juice, light soy sauce, mirin, dashi, rice vinegar, and sugar until the sugar dissolves. If using yuzu juice, add it here in place of part of the lemon/lime for a more authentic ponzu profile.

Taste and adjust: Taste the ponzu and adjust balance — add a little more soy if you want saltiness, more citrus if brighter acidity is desired, or a pinch more sugar to round the edges. The finished sauce should be bright, savory, and balanced.

Rest: Let the sauce rest for 5–10 minutes at room temperature to allow flavors to meld. Chill briefly if you prefer the sauce cold.

Dress the fish: Just before serving, spoon 1–1.5 teaspoons of ponzu over each yellowtail slice (or lightly drizzle the sauce across the entire plate). Alternatively, serve the ponzu in a small dipping bowl on the side.