Prepare the brown rice according to package instructions and set aside to cool slightly.
Slice the sushi-grade ahi tuna into bite-sized 1/2-inch cubes using a sharp knife.
In a medium glass bowl, whisk together the tamari, sesame oil, rice vinegar, and grated fresh ginger.
Add the tuna cubes and sliced green onions to the marinade, tossing gently to coat every piece.
Let the tuna marinate in the refrigerator for 10 to 15 minutes to absorb the flavors.
Thinly slice the cucumber, radish, and avocado.
Assemble the bowl by placing the cooked brown rice at the base, then topping with the marinated tuna.
Arrange the cucumber, radish, edamame, and avocado slices around the tuna.
Garnish with black sesame seeds and serve immediately while the tuna is chilled.