Cook the brown rice according to package directions and allow it to cool slightly before assembling.
Pat the sushi-grade yellowfin tuna dry with a paper towel and cut into uniform 1/2-inch cubes.
In a medium glass bowl, whisk together the tamari, toasted sesame oil, and freshly grated ginger.
Add the tuna cubes to the bowl and toss gently to coat, then let marinate in the refrigerator for 5 to 10 minutes.
Roughly chop the macadamia nuts and toast them in a small dry skillet over medium heat until golden and fragrant.
Slice the cucumber into thin half-moons and the radish into translucent rounds.
Place the cooled brown rice in the center of a shallow bowl.
Top the rice with the marinated tuna, edamame, cucumber, and radish.
Garnish the bowl with the toasted macadamia nuts, sliced green onions, black sesame seeds, and a pinch of sea salt.