In a large mixing bowl, combine the soy sauce, sliced green onion, lime juice, toasted sesame oil, sesame seeds, and sriracha; whisk until blended.
Add the diced sushi-grade tuna to the bowl and gently fold the tuna with the marinade until evenly coated, taking care not to break up the pieces.
Cover and refrigerate the marinated tuna for 15 minutes, or up to 1 hour maximum.
Prepare your chosen rice according to package directions.
Assemble bowls by dividing rice among bowls, arranging pickled vegetables and fresh vegetables as desired, and topping with the marinated tuna.
Garnish each bowl with additional sesame seeds, sliced green onion, furikaki, or spicy mayo, and serve with optional lime wedges or other additions.