Pat the sushi-grade tuna dry with a paper towel and cut into even 1/2-inch cubes using a very sharp knife.
In a medium glass bowl, whisk together the tamari, sesame oil, freshly grated ginger, lime juice, and sriracha to create the marinade.
Add the tuna cubes to the bowl and toss gently with a spoon until every piece is coated; let it marinate in the refrigerator for 10 minutes.
Place the warm, cooked brown rice in the bottom of a wide serving bowl as the base.
Arrange the marinated tuna, sliced avocado, shelled edamame, sliced cucumber, and sliced radishes in separate sections over the rice.
Drizzle any remaining marinade from the bowl over the top and garnish with sliced green onions, sesame seeds, sea salt, and black pepper.