In a medium bowl, add all the ingredients for the marinade: soy sauce, rice vinegar, sesame oil, ginger paste, mirin and, if using, the jalapeño, and green onions.
Whisk marinade ingredients together until well incorporated.
Taste! If you want a bit more bite, add more rice vinegar. Or if you want more ginger, add another teaspoon of ginger paste.
When the marinade tastes perfect to you, add the ahi.
Use a spoon to mix the ahi with the marinade. Cover and store in the refrigerator for at least 30 minutes and up to 1 hour.
While the ahi marinates, prep all the toppings and cook the rice.
***RICE NOTE: If you don't have a rice cooker -- it's definitely time!! A rice cooker makes cooking rice so much easier!!