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, toasted sesame oil, rice vinegar, and freshly grated ginger.
Add the cubed tuna to the marinade and toss gently to coat; let it sit for 10 minutes in the refrigerator.
Prepare the base by placing the warm cooked brown rice in a serving bowl.
Thinly slice the English cucumber, radishes, and scallion, and pit and slice the avocado.
Arrange the marinated tuna, edamame, cucumber, radishes, and avocado over the rice.
Garnish with sliced scallions and black sesame seeds, then sprinkle with a pinch of sea salt before serving.