Place the tuna in a large bowl. In a smaller bowl, whisk together the soy sauce, oil, vinegar, sesame seeds, ginger and garlic. Pour over the tuna and toss to coat. Stir in the green onion. Cover the bowl with plastic wrap and place it in the fridge to chill for 10 minutes.
Right before serving, stir in the cubed avocado and the pineapple.
To make the bowls, place 1 cup of rice in the bowl. Cover with a handful of cucumbers (I like to toss mine in toasted sesame oil), seaweed salad (see post above for my tips, I buy a packaged one and follow the directions) extra sliced avocado and a big scoop of the tuna and marinade. Top with extra sesame seeds. This is wonderful eaten with tortilla chips!