Cook the brown rice according to package instructions and let it cool slightly to room temperature.
Pat the ahi tuna dry with a paper towel and cut it into 1/2-inch uniform cubes using a very sharp knife.
In a medium glass bowl, whisk together the tamari, toasted sesame oil, grated fresh ginger, lime juice, and sea salt.
Add the cubed tuna to the marinade, tossing gently to coat, and let it marinate in the refrigerator for 10 minutes.
Prepare the vegetables by slicing the cucumber into thin rounds, thinly slicing the radishes, and cubing the avocado.
Assemble the bowl by placing the cooked brown rice at the bottom, then topping with the marinated tuna, edamame, cucumber, radish, and avocado.
Garnish the bowl with black sesame seeds and serve immediately while the tuna remains chilled.