In a large bowl, whisk together soy sauce, rice vinegar, sesame oil, ginger, red pepper flakes, green onions, and sesame seeds. Add tuna and toss to coat. Refrigerate for at least 15 minutes or up to 1 hour.
To serve, add rice to the bottom of four bowls. Top with tuna and toppings of your choice. Garnish with green onions and sesame seeds before serving.