Cook ingredients - Add kombu, katsuobushi, mirin, and rice vinegar in a small saucepan and cook on low until the mixture just simmers. Give it a stir, remove from heat, and allow the mixture to cool. The mixture will smell sweet and smoky, and the Kombu will soften.
Strain - Once the mixture cools after about 10 minutes, strain it to remove the katsuobushi and kombu and pour the mixture into a jar.
Shake - Add the soy sauce and yuzu juice to the jar. Secure the lid on the jar and give the mixture a good shake until combined.
Mix ponzu and garlic - Combine yuzu ponzu, soy sauce, and garlic in a small bowl and stir to combine. Let the mixture sit while you prepare the hamachi.
Slice fish - Slice hamachi against the grain at a 45° angle. Be sure to pick how large you want the slices ahead of time and cut in one smooth motion. A standard thickness is about ⅜-inch thick. Then arrange the slices on the plate and add a slice of jalapeno to each hamachi slice.
Drizzle sauce and serve - Use a spoon to drizzle the yuzu ponzu over the hamachi. Garnish with Ikura and lemon slices and serve with wasabi.
