In a medium glass bowl, whisk together soy sauce, brown sugar, rice wine vinegar, sesame oil, fish sauce, ginger and garlic until combined.
Add cubes of fresh ahi tuna and stir to combine. Cover bowl and refrigerate at least one hour. In a medium glass bowl, combine chopped vegetables and rice wine vinegar; toss to combine.
Cover bowl and refrigerate at least one hour. (This is an optional step but adds a ton of flavor.)
To assemble bowls: Spoon rice into four bowls. Distribute pickled vegetables, mango, avocado, and marinated tuna among bowls. Sprinkle with green onions and sesame seeds. Serve with spicy mayo, if desired.