Cook the brown rice according to package instructions and set aside to cool slightly.
Pat the 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, grated fresh ginger, lime juice, and red pepper flakes.
Add the tuna cubes to the marinade, tossing gently to ensure every piece is coated, and let it marinate for 10 minutes in the refrigerator.
Prepare the produce by thinly slicing the cucumber and radishes, dicing the avocado, and chopping the scallion.
Place the cooked brown rice in the center of a serving bowl.
Arrange the marinated tuna, edamame, cucumber, radishes, and avocado in sections over the rice.
Garnish the bowl with chopped scallions, black sesame seeds, and a pinch of sea salt before serving immediately.