Peel, seed and finely dice cucumber. Dice avocado into similar-sized pieces as the tuna and toss gently with a light squeeze of lime to slow browning only if not serving immediately. Finely slice scallions and mince shallot and cilantro if using.

Yuzu-Tamari Dressing

In a small bowl or jar combine yuzu juice, tamari, sesame oil, neutral oil, rice vinegar, sugar (or agave), grated ginger, black pepper, and kosher salt. Whisk or shake vigorously until sugar dissolves and the dressing emulsifies slightly. Taste and adjust: add up to 1/2 tsp more yuzu for brighter acidity or 1/2 tsp more tamari for saltiness.

Reserve about 1 teaspoon of the dressing for drizzling at the end; fold the remainder into the bowl with the diced tuna, mixing gently but thoroughly to coat all pieces. Let sit on ice or refrigerated surface for 3–5 minutes to marry flavors (do not marinate longer than 10 minutes to preserve texture).

Finish & Garnish

Fold the cucumber, scallions, and shallot gently into the dressed tuna. If using avocado and cilantro, fold them in last and very gently to avoid mashing the avocado.

Using a ring mold (2.5–3 inches) or spoon, portion the tartare onto chilled plates: place the mold, pack lightly with the mixture and lift straight up for a neat tower. Alternatively, spoon into small bowls or onto wonton chips for canapé service.

Top each portion with a small spoonful (about 1/2–1 tsp) of tobiko, then scatter toasted sesame seeds and microgreens or shiso leaves. Drizzle reserved dressing around the plate or a few drops atop the tartare. Serve immediately with wonton chips or sliced baguette and lime wedges on the side.

Storage: Tuna tartare is best served immediately. If needed, keep components separate and assemble within 1 hour. Do not store formed tartare for more than 2 hours in the refrigerator.