Pat the sushi-grade tuna dry with paper towels. Trim any silver skin and membranes. Cut into uniform 1/2-inch (about 1.2 cm) dice and place in a chilled bowl.

Make the Nikkei marinade

In a separate bowl combine fresh lime juice, lemon juice, soy sauce, mirin, sesame oil, grated ginger, and minced garlic. Whisk until combined. Taste and adjust balance of citrus and soy: it should be bright with a hint of umami.

Assemble ceviche

Add the thinly sliced red onion, aji amarillo paste, half the chopped cilantro, and half the sliced scallions to the marinade and stir. Pour the marinade over the diced tuna and gently toss to coat. Season with 1 1/2 teaspoons kosher salt and 1/2 teaspoon black pepper, then taste and adjust salt if necessary. Allow to sit in the refrigerator for 8–12 minutes — the tuna is not meant to be 'cooked' through like fish cured for long periods; this timing softens edges while keeping the center bright and sashimi-like.

Finish and plate

Just before serving, fold in the toasted sesame seeds and the remaining chopped cilantro and scallions. Check seasoning and add a squeeze of fresh lime if needed. Portion the ceviche onto chilled plates or small bowls.

Avocado Mousse

Halve and pit the avocados. Scoop flesh into a food processor or blender. Add Greek yogurt, lime juice, olive oil, salt, and white pepper.

Blend until smooth and silky, scraping down sides as needed. If the mousse is too thick, add cold water 1 tbsp (15 ml) at a time until you reach a light, pipeable consistency suitable for dollops or to pipe into rings. Taste and adjust seasoning.

Transfer mousse to a piping bag or spoon into a small bowl and chill briefly while finishing ceviche.

Chips & Garnishes

Arrange chips on a serving platter or alongside individual servings. If using plantain chips, warm briefly in a 150°C (300°F) oven for 3–4 minutes to refresh crispness.

Plate components: place a quenelle or piped dollop of avocado mousse on each plate, set a portion of tuna nikkei ceviche next to it, and tuck chips alongside. Garnish with microgreens or baby cilantro, thin radish slices, and lime wedges. Serve immediately so chips remain crisp.