Cut the sashimi-grade ahi tuna into 1/2-inch cubes using a very sharp knife and set aside in a medium glass bowl.
In a small mixing bowl, whisk together the tamari, toasted sesame oil, rice vinegar, and freshly grated ginger until well combined.
Pour the dressing over the tuna cubes and toss gently to ensure every piece is coated, then let it marinate in the refrigerator for 10 to 15 minutes.
While the tuna marinates, prepare the base by placing the warm cooked brown rice into a serving bowl.
Thinly slice the cucumber and avocado, and ensure the edamame is shelled and ready for assembly.
Arrange the marinated tuna, cucumber slices, edamame, and avocado over the rice in neat sections.
Garnish the bowl with thinly sliced green onions and a sprinkle of sesame seeds before serving immediately.