Hawaiian Kanpachi Sashimi with Sweet Onion Ponzu

Ingredients

• 1 (2-inch) square kombu (dried kelp)
• 2 tablespoons bonito flakes (shaved katsuobushi, also called hana-katsuo)
• 1/2 cup shoyu (soy sauce)
• 1/4 cup freshly squeezed lemon juice (from 1 to 2 lemons)
• 1/4 cup freshly squeezed lime juice (from 2 to 3 limes)
• 1/2 cup finely diced sweet onion, rinsed and drained well
• 1 1/2 teaspoons minced peeled fresh ginger
• 1 tablespoon finely chopped fresh shiso (from 2 to 3 leaves)
• 9 ounces skinless Hawaiian Kanpachi fillet
• Flaky sea salt, such as Maldon
• Extra-virgin olive oil (or lemon olive oil, see Tip)