Keep the tuna very cold. Trim away any sinew. Using a very sharp knife, cut the tuna into 1/4 inch (6 mm) dice. Place diced tuna in a chilled bowl and return to refrigerator while you prepare the other components. Chilling helps keep the fish firm and prevents it from releasing too much liquid.

Prepare aromatics and mix-ins

Very finely mince the red onion, jalapeño (if using), capers, and cilantro (or chives). Thinly slice the scallions. If using avocado, dice it into small 1/4 inch cubes and hold in a bowl with a squeeze of lime to slow browning.

Make the dressing

In a small bowl, whisk together the soy sauce, sesame oil, fresh lime juice, olive oil, Dijon mustard, and honey (if using). Taste and adjust: add more lime for brightness, more sesame oil for nuttiness, or a bit more soy for saltiness. The dressing should be balanced and slightly more assertive than a typical salad dressing, because it will coat raw fish.

Combine tartare

Remove the tuna from the refrigerator. Add the minced red onion, scallions, capers, jalapeño, and chopped herbs to the tuna. Pour about two-thirds of the dressing over the mixture and gently fold with a rubber spatula or spoon until evenly coated. Taste and add more dressing if needed. Season with sea salt and freshly ground black pepper — add salt sparingly since soy and capers are salty. If using avocado, gently fold it in last to avoid mashing.

Rest and chill briefly

Cover loosely and chill the assembled tartare in the refrigerator for 5–10 minutes to let flavors marry, but do not let it sit too long (no more than 20–30 minutes) to preserve texture and freshness of the tuna.

Plate and garnish

Use a round ring mold (2.5–3 inches) or a small ramekin placed upside down to shape portions. Spoon the tuna tartare into the mold and press lightly to compact. Release the mold and garnish the top with toasted sesame seeds and microgreens or small herb leaves. Drizzle remaining dressing around the plate or a few drops on top for shine.

Serve

Serve immediately with crispy wonton chips, crostini, or cucumber slices. Provide small forks or spoons so guests can scoop without crushing the texture. Leftovers should be kept chilled and consumed within 24 hours.