Cut the sushi-grade ahi tuna into 1/2-inch cubes using a very sharp knife.
In a medium glass bowl, whisk together the tamari, toasted sesame oil, rice vinegar, grated fresh ginger, sea salt, and black pepper.
Add the tuna cubes to the marinade and toss gently to coat; cover and refrigerate for 10 to 15 minutes.
While the tuna marinates, thinly slice the cucumber into rounds, slice the radishes into matchsticks, and finely chop the green onion.
Place the warm cooked brown rice in the bottom of a serving bowl.
Arrange the marinated tuna, shelled edamame, sliced cucumber, radishes, and diced avocado in sections over the rice.
Drizzle any remaining marinade from the tuna bowl over the top and garnish with sesame seeds and green onions.