Pat the ahi tuna dry with a paper towel and cut it into 1/2-inch uniform cubes.
In a small glass bowl, whisk together the tamari, toasted sesame oil, rice vinegar, sea salt, and black pepper.
Add the tuna cubes to the marinade and toss gently to coat; let it sit in the refrigerator for 10 minutes.
While the tuna marinates, dice the mango, pineapple, and cucumber into small pieces, and thinly slice the green onion.
Place the cooked brown rice in the center of a serving bowl.
Arrange the marinated tuna, diced mango, pineapple, cucumber, and avocado slices on top of the rice.
Garnish the bowl with sesame seeds and sliced green onions before serving immediately.