Cook the brown rice according to package instructions and let it cool slightly to room temperature.
Using a sharp knife, carefully dice the sushi-grade ahi tuna into uniform half-inch cubes.
In a small glass bowl, whisk together the tamari, sesame oil, grated ginger, lime juice, sea salt, and black pepper.
Place the tuna cubes in a medium bowl and pour the marinade over them, tossing gently to coat every piece.
Allow the tuna to marinate in the refrigerator for 10 to 15 minutes to let the flavors penetrate.
Prepare the vegetables by dicing the cucumber, shelling the edamame, and slicing the scallion thinly.
Scoop the cooked brown rice into a serving bowl as the base.
Arrange the marinated tuna, cucumber, edamame, and avocado slices over the rice.
Garnish the bowl with black sesame seeds and sliced scallions before serving immediately.