Cook the white rice according to package directions; once finished, fluff with a fork and set aside to cool slightly to room temperature.
Using a sharp chef's knife, carefully dice the chilled sushi-grade ahi tuna into uniform 1/2-inch cubes.
In a medium glass bowl, whisk together the tamari, toasted sesame oil, and freshly grated ginger until well combined.
Gently fold the tuna cubes into the marinade, ensuring each piece is coated, and refrigerate for 10-15 minutes.
While the tuna marinates, toss the diced mango, cucumber, and sliced green onion with lime juice in a separate small bowl.
Slice the avocado into thin wedges and season lightly with sea salt and black pepper.
To serve, divide the rice into a bowl, top with the marinated tuna, and spoon the tropical salsa over the side.
Finish the bowl with the avocado wedges and a sprinkle of toasted sesame seeds for a crunch.