Pat the ahi tuna dry with a paper towel and cut into uniform 1/2-inch cubes using a sharp knife.
In a medium glass bowl, whisk together the tamari, rice vinegar, toasted sesame oil, grated fresh ginger, and sea salt.
Add the cubed tuna to the marinade, tossing gently to coat every piece, and let it sit in the refrigerator for 10 to 15 minutes.
While the tuna marinates, thinly slice the cucumber into half-moons, slice the radishes into rounds, and finely chop the green onion.
Place the warm cooked brown rice in the bottom of a serving bowl.
Arrange the marinated tuna, sliced avocado, cucumber, radishes, and edamame on top of the rice.
Garnish the bowl with chopped green onions, sesame seeds, and nori strips before serving immediately.