Cook the brown rice according to package instructions and allow it to cool to room temperature.
Using a sharp knife, cut the sashimi-grade ahi tuna into uniform 1/2-inch cubes.
In a medium glass bowl, whisk together the tamari, sesame oil, lime juice, and freshly grated ginger.
Add the tuna cubes to the marinade, tossing gently to coat, and let it rest in the refrigerator for 10 minutes.
Thinly slice the cucumber, scallion, and avocado, and ensure the edamame is thawed if using frozen.
Place the cooked brown rice in the bottom of a serving bowl.
Top the rice with the marinated tuna, cucumber slices, edamame, and avocado.
Garnish the bowl with sliced scallions, sesame seeds, and shredded nori before serving immediately.