Pat the ahi tuna dry with a paper towel and cut into 1/2-inch cubes using a sharp knife.
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 let sit in the refrigerator for 10 minutes.
Thinly slice the radishes, cucumber, and green onion while the tuna marinates.
Place the warm cooked brown rice in the center of a serving bowl.
Arrange the marinated tuna, edamame, sliced avocado, radishes, and cucumber over the rice.
Garnish the bowl with sliced green onions and sesame seeds before serving immediately.