Cook the soba noodles in boiling water according to package directions, then rinse immediately with cold water and drain well to maintain a firm texture.
Pat the ahi tuna dry with a paper towel to ensure the marinade adheres, then cut the fish into uniform 1/2-inch cubes.
In a medium glass bowl, whisk together the tamari, sesame oil, and rice vinegar until well combined.
Gently fold the tuna cubes into the marinade, ensuring every piece is coated, and let it sit in the refrigerator for 5 to 10 minutes.
While the tuna marinates, thinly slice the cucumber and radishes into rounds, and finely chop the scallion.
Place the chilled soba noodles in the base of a serving bowl and arrange the marinated tuna on top.
Add the sliced avocado, cucumber, and radishes to the bowl in neat sections for a beautiful presentation.
Garnish the bowl with the sliced scallions, sesame seeds, and a finishing pinch of sea salt before serving immediately.