Prepare the brown rice according to package instructions and set aside to cool slightly.
Pat the ahi tuna dry with a paper towel and cut into 1/2-inch cubes using a sharp chef's knife.
In a medium glass bowl, whisk together the tamari, toasted sesame oil, rice vinegar, and freshly grated ginger.
Gently fold the cubed tuna into the marinade along with the sea salt and black pepper, then refrigerate for 10 minutes.
While the tuna marinates, dice the fresh mango, slice the cucumber into half-moons, and pit the avocado.
Place the cooked brown rice in the base of a shallow bowl and arrange the marinated tuna, mango, cucumber, shelled edamame, and avocado on top.
Finish the bowl by garnishing with thinly sliced green onions and toasted sesame seeds before serving immediately.