Cook the brown rice according to package instructions and set aside to cool slightly.
Slice the raw ahi tuna steak into 1/2-inch bite-sized cubes using a sharp knife.
In a small glass bowl, whisk together the tamari, toasted sesame oil, rice vinegar, and grated fresh ginger.
Toss the tuna cubes in the marinade and let them sit for 5-10 minutes in the refrigerator to absorb the flavors.
Thinly slice the cucumber, radishes, and green onion, and pit and slice the avocado.
Assemble the bowl by placing the brown rice at the base, followed by the marinated tuna.
Arrange the cucumber, radishes, edamame, and avocado slices around the tuna.
Garnish the entire bowl with sliced green onions and black sesame seeds before serving.