Cook the brown rice according to package directions and let it cool completely to room temperature.
Slice the fresh ahi tuna into bite-sized 0.5-inch cubes using a very sharp knife.
In a small glass bowl, whisk together the tamari, sesame oil, and freshly grated ginger to create the marinade.
Place the tuna cubes in the marinade, toss gently to coat every piece, and let sit in the refrigerator for 10 minutes.
Dice the mango, avocado, and cucumber into uniform pieces and thinly slice the green onion.
Arrange the cooled rice in the base of a bowl and top with the marinated tuna, edamame, mango, avocado, and cucumber.
Sprinkle with toasted sesame seeds and sliced green onions before serving immediately.