Keep tuna cold until ready. Pat the tuna dry with paper towels. Trim away any sinew. Cut tuna into 1/4-inch dice for a tender, slightly chunky tartare texture.

Make seasoning

In a medium bowl combine soy sauce, sesame oil, rice vinegar, grated ginger, minced shallot, lime juice, Sriracha (if using), toasted sesame seeds, sliced scallion, kosher salt, and freshly ground black pepper. Whisk briefly to combine and taste; adjust salt or acidity as needed.

Combine

Add the diced tuna to the seasoning and gently fold with a rubber spatula until evenly coated. Do not overmix; you want the tuna pieces intact. Refrigerate the mixture for 5–10 minutes to allow flavors to marry while you prepare garnishes.

Accompaniments & Assembly

Prepare lettuce and garnishes

Separate gem lettuce into individual leaves, keeping them intact and cold. Julienne the ninja (black) radish into thin matchsticks and pat dry. Thinly slice or chiffonade shiso leaves. If using microgreens or chives, have them ready. Lightly toss the radish with a tiny pinch of salt to soften edge flavor if desired, then blot any moisture.

Plate tartare

To assemble individual portions, either spoon about 1.5 to 2 oz (roughly 3–4 tbsp) of the tuna tartare into each gem lettuce leaf, or use a ring mold on a plate and press gently to shape 3–4 oz portions. If using crackers, place a small mound on each cracker instead of lettuce. Drizzle a few drops of neutral oil over each portion for sheen.

Garnish

Top each portion with a small pinch of julienned ninja radish, a shiso ribbon, and a light dusting (about 1/8 tsp) of togarashi. Finish with microgreens or chives and an extra grind of black pepper if desired. Serve immediately so lettuce stays crisp.

Serving & Notes

Serve the tuna tartare chilled on a platter with remaining gem leaves and crackers on the side. Provide small spoons or let guests pick up the lettuce cups. Leftover seasoned tuna should be stored in an airtight container and consumed within 24 hours.