Slice the tuna sashimi into bite-sized chunks
Add the dashi, mirin, soy sauce, sesame oil and salt into a marinating container with the tuna sashimi
Mix the marinade with the tuna and keep in refrigerator for at least 30 minutes
To make the rice vinegar, mix the 1 tbsp sugar, 3 tbsp vinegar, and 0.25 tbsp salt in a small mixing bowl or cup
Slowly add the rice vinegar into the rice and mix well. Mix the rice so as not to crush the rice.
Allow the rice to cool down and top with the marinated tuna. Finish with a green onion garnish and a dollop of wasabi. Enjoy!