Cook the brown rice according to package instructions and allow it to cool to room temperature.
Using a sharp knife, cut the yellowfin tuna into uniform 1/2-inch cubes and pat dry with a paper towel.
In a small glass bowl, whisk together the tamari, sesame oil, rice vinegar, and freshly grated ginger.
Add the tuna cubes to the marinade, tossing gently to coat, and refrigerate for 10 minutes.
Thinly slice the cucumber, radishes, and green onion, and dice the avocado.
Assemble the bowl by placing the cooled brown rice at the base.
Arrange the marinated tuna, edamame, cucumber, radishes, and avocado in sections over the rice.
Drizzle any remaining marinade over the top and garnish with sliced green onions and sesame seeds.