Place sushi-grade tuna on a clean cutting board. Using a very sharp chef's knife, trim any sinew and cut the tuna into 1/4-inch dice. Transfer the diced tuna to a chilled bowl and keep refrigerated until ready to dress.

Dice the avocado

Halve, pit, and peel the avocado. Cut into 1/4-inch dice to match the tuna. To prevent browning, toss gently with 1/2 tablespoon of the lemon juice and set aside in the refrigerator in a small covered bowl.

Make the dressing

In a small bowl combine 2 tablespoons soy sauce, 1 teaspoon toasted sesame oil, 1 tablespoon fresh lemon juice, 1 teaspoon lemon zest, 1 tablespoon finely diced shallot, 1 teaspoon neutral oil, 1 teaspoon sesame seeds, 1 teaspoon fine sea salt, and 1/4 teaspoon freshly ground black pepper. Whisk briefly to emulsify and taste; adjust seasoning (add up to 1/2 tablespoon more soy or a pinch more salt) if needed.

Dress the tuna

Remove the tuna from the refrigerator and gently fold the dressing into the diced tuna until evenly coated. Taste and adjust: if it needs more brightness, add a bit more lemon juice; if salt is low, add a few drops more soy. Keep chilled.

Finish and fold with avocado

Just before plating, fold the diced avocado and chopped chives/green onion into the dressed tuna gently so the avocado holds shape. Sprinkle the toasted sesame seeds over the mixture and give a final gentle toss. Check seasoning once more and add a small pinch of salt or pepper if desired.

Crostini & Garnish

Prepare crostini

Preheat oven to 375°F (190°C). Arrange baguette slices in a single layer on a baking sheet. Brush each slice lightly with extra-virgin olive oil (or spread a thin layer of softened butter for a richer flavor). Bake for 6–9 minutes, or until edges are golden and crisp. Remove and cool slightly.

Plate and garnish

Using a ring mold (optional) or two spoons, portion 2–3 tablespoons of tuna tartar onto each crostini or place the tartar in a small bowl for communal service. Garnish with microgreens or small herb leaves and an additional pinch of toasted sesame seeds and lemon zest if you like. Serve immediately with lemon wedges on the side.