Pat the ahi tuna dry with paper towels. Trim away any sinew. Cut the tuna into 1/4- to 1/2-inch small dice for a tartare-like texture or into 1/2-inch cubes if you prefer chunkier pieces.

Make marinade

In a small bowl, whisk together 2 tablespoons light soy sauce, 1 teaspoon sesame oil, 1 teaspoon fresh lemon juice, 1/2 teaspoon grated ginger, 1/2 teaspoon minced garlic, 1/4 teaspoon kosher salt and 1/8 teaspoon freshly ground black pepper until combined.

Marinate tuna briefly

Place the diced tuna in a medium bowl and pour the marinade over it. Gently fold to coat evenly. Let sit at room temperature for 5–10 minutes (do not marinate longer than 15 minutes to preserve texture and flavor).

Sesame crust and basil

Toast sesame seeds (if not pre-toasted)

If your sesame seeds are untoasted, warm a small dry skillet over medium heat. Add 1 tablespoon white and 1 tablespoon black sesame seeds and toast, stirring frequently, until fragrant and lightly golden, 1–2 minutes. Transfer to a plate to cool.

Chop basil and combine with seeds

Stack the basil leaves, roll them tightly and thinly slice (chiffonade) or finely chop. In a small bowl combine the toasted sesame seeds and the chopped basil. Add 1 teaspoon thinly sliced chives if using. This mixture will be used to coat and garnish the tuna.

Coat tuna

Drain any excess liquid from the marinated tuna by gently pressing with a slotted spoon or letting it sit briefly in a sieve. Toss the tuna gently with the sesame-basil mixture so the pieces are lightly coated on all sides. Use only as much coating as adheres—reserve extra for garnish.

Assemble and finish

Plate base

Arrange optional cucumber slices or ribbons on serving plates or place toasted baguette slices/wonton crisps if serving as canapés. Drizzle a thin film of olive oil over the cucumber or toasts (about 1 teaspoon spread among servings).

Spoon the sesame-crusted tuna onto the prepared base (or mound on a chilled plate). Aim for about 1/4 lb tuna per two servings as portioned here; divide evenly.

Add sprouts and garnish

Top each portion with a small handful (about 2 tablespoons) of mixed sprouts. Sprinkle the remaining sesame-basil mixture over the top for texture. Lightly scatter the sliced chives if using.

Sprinkle with rose water

Using a dropper or measuring spoon, very lightly sprinkle about 1/8–1/4 teaspoon food-grade rose water evenly over the plated tuna (a little goes a long way). Avoid drenching—this should impart a subtle floral note.

Finish and serve

Season lightly with a pinch of fine sea salt and a crack of black pepper. Serve immediately so tuna remains fresh and cool. If desired, offer extra soy sauce or citrus wedges at the table.