Prepare the brown rice according to package directions and allow it to cool to room temperature.
Cut the ahi tuna into 1/2-inch cubes using a very sharp knife.
In a small bowl, whisk together the tamari, toasted sesame oil, and grated fresh ginger.
Combine the tuna cubes and sliced green onion in a bowl, then pour the tamari mixture over them and toss gently to coat.
Steam the shelled edamame until tender and bright green, then set aside to cool.
Slice the cucumber into rounds, the radishes into thin half-moons, and the avocado into small cubes.
Layer the rice in a serving bowl and arrange the marinated tuna, edamame, cucumber, radishes, and avocado on top.
Sprinkle with black sesame seeds and sea salt before serving immediately.