Place the yellowtail fillet in the coldest part of the refrigerator for 15–30 minutes to firm it slightly (this makes cleaner slicing). Pat dry with paper towels.

Trim and slice

Using a very sharp sashimi or long thin knife, trim away any remaining silver skin or sinew. Hold the knife at about a 15–20° angle and slice the fish into 1/8–1/4 inch (3–6 mm) thick slices, cutting across the grain. Aim for uniform pieces about 2–3 inches long.

Season slices

Lightly season both sides of each yellowtail slice with a pinch of kosher salt and a tiny grind of black pepper. Arrange slices on a chilled serving plate, slightly overlapping or fanned.

Add garnishes

Top each slice or place alongside with 1–2 thin jalapeño rounds (remove seeds for less heat), scatter microgreens or cilantro, sprinkle sliced scallion and toasted sesame seeds. Place lemon or yuzu wedges on the plate for optional brightening.

If serving family-style, keep the fish chilled until just before serving. Sashimi should be eaten within 1–2 hours of slicing for best texture and safety.

Ponzu Sauce

Combine liquid ingredients

In a small bowl or jar, whisk together the soy sauce, fresh citrus juice (yuzu or lemon/lime), mirin, rice vinegar and dashi until homogenous.

Season and balance

Add the granulated sugar (if using) and the grated ginger. Taste and adjust: if too salty, add an extra 1/2–1 tbsp citrus; if too acidic, add another 1/2 tsp mirin or a drop more sugar. Stir in the sesame oil for a rounded finish.

Rest and strain (optional)

Let the ponzu sit 5–10 minutes to meld flavors. For a very clear sauce, strain through a fine sieve to remove ginger bits; otherwise serve as-is for texture.

Pour the ponzu into a small dipping bowl or drizzle lightly over the plated yellowtail just before serving. Provide extra ponzu on the side for dipping.