Prepare the brown rice according to package instructions and set aside to cool slightly.
Using a sharp chef's knife, cut the sushi-grade ahi tuna into uniform 1/2-inch cubes.
In a medium glass bowl, whisk together the tamari, toasted sesame oil, rice vinegar, and freshly grated ginger.
Add the tuna cubes to the bowl and toss gently to ensure every piece is coated in the marinade; let it sit for 5 to 10 minutes.
Slice the cucumber into thin rounds, slice the radishes into matchsticks, and finely chop the scallion.
Roughly chop the macadamia nuts into small, crunchy pieces.
Place the cooked brown rice in the center of a serving bowl.
Arrange the marinated tuna, edamame, cucumber, and radishes in sections over the rice.
Top the bowl with the chopped macadamia nuts, sliced scallions, and a sprinkle of sesame seeds before serving chilled.