Prepare the Matsuhisa Dressing: In a medium bowl, combine minced white onion, soy sauce, rice vinegar, dry mustard, and water. Whisk until the salt dissolves completely, then add grapeseed and toasted sesame oils, mixing until wonderfully blended.

Make the Yuzu Soy Sauce: In another medium bowl, combine yuzu citrus juice, soy sauce, and olive oil. Mix well using a spoon until all ingredients harmoniously meld together.

Create the Yuzu Olive Oil: In a small bowl, mix yuzu citrus juice with olive oil. Stir well to create a vibrant, citrusy drizzle.

Cook the Nobu-Style Saikyo Miso: In a medium saucepan, bring sake and mirin to a boil over high heat. Boil for 20 minutes. Stir in miso paste until dissolved, then add sugar; stir until it dissolves. Allow to cool to room temperature.

Combine for the Karashi Sumiso: In a medium bowl, blend the cooled Nobu-Style Saikyo Miso with the elements needed for your Karashi Sumiso.

Sear the Tuna Tataki: Season the tuna fillet with kosher salt and black pepper. Heat olive oil in a skillet over high heat and sear the tuna for 5 seconds on each side. Plunge it into ice water for 10 seconds, then pat dry.

Slice the Sashimi: On a clean surface, thinly slice the seared tuna fillet into 10 rectangular slices, repeating with the salmon, yellowtail, and fluke.

Assemble the Spoons: Roll a slice of fish, drizzle sauce over it, and place it on a spoon or plate. Repeat for remaining sashimi. Serve immediately.