Prepare the brown rice according to package instructions and allow it to cool slightly to room temperature.
Using a very sharp knife, cut the sushi-grade ahi tuna into uniform 1/2-inch cubes.
In a small glass bowl, whisk together the tamari, toasted sesame oil, rice vinegar, and freshly grated ginger.
Add the tuna cubes to the marinade, tossing gently to ensure every piece is coated, and let it marinate for 5 to 10 minutes in the refrigerator.
Thinly slice the cucumber into rounds and dice the avocado into small chunks.
Place the cooked brown rice in the center of a serving bowl.
Arrange the marinated tuna, cucumber slices, shelled edamame, and avocado on top of the rice.
Garnish the bowl with thinly sliced green onions, sesame seeds, and a light sprinkle of sea salt before serving.