Pat the sushi-grade ahi tuna dry with a paper towel and cut into uniform 1/2-inch cubes.
In a medium glass bowl, whisk together the tamari, sesame oil, rice vinegar, and freshly grated ginger.
Add the tuna cubes to the bowl, tossing gently to coat every piece in the marinade, and set aside for 10 minutes.
Steam the cauliflower rice until tender, then mix it thoroughly with the cooked brown rice to create a high-volume base.
Slice the cucumber into thin rounds and pit the avocado, slicing one quarter into thin wedges.
Place the rice mixture into a serving bowl and top with the marinated tuna, edamame, cucumber, and avocado.
Garnish the bowl with sliced green onions, sesame seeds, and a light sprinkle of sea salt before serving.