|Tuna fillet||7 Ounce (Fresh Or Frozen, Raw Pieces Of Sashimi Quality, 200 Grams)|
|Mild green onions||7|
|Wasabi horseradish||1/2 Teaspoon|
|Soy sauce||2 Teaspoon|
|Rice vinegar||2 Teaspoon|
|For miso and vinegar dressing|
|White miso||4 Tablespoon|
|Secondary bonito stock||2 Tablespoon|
|Rice vinegar||3 Tablespoon|
|Hot mustard/Any hot mustard that is not sweet / vinegary||1 Teaspoon (Yellow Japanese, Karashi)|
1. Immerse white end of green onion into boiling salted water until it softens (about 1 minute). Immerse whole onion and parboil. Transfer immediately to cold water and quickly cool. Drain. Arrange onion side by side on a cutting board. Remove the bitter substance from the middle of the onion by running the side of a knife over onion, forcing the substance out at the top. Cut green onion into 1-inch (2 1/2-cm) pieces. Sprinkle with 2 tsps rice vinegar.
2. Cut tuna into bite-sized cubes. Combine with wasabi horseradish and soy sauce and coat well.
3. MISO AND VINEGAR DRESSING: In a small saucepan combine miso, bonito stock, sugar, and mirin. Stir over medium-high heat until mixture reaches boiling point. Remove from heat and let cool. When cool, add rice vinegar and mustard.
4. Squeeze out any excess moisture from onion. Combine onion, tuna, and dressing. Arrange and serve.