Simply cube the tuna and combine it with sesame oil, soy sauce, green onions, sweet onions, and ginger in a bowl.
Mix everything well to ensure it's coated evenly.
Cover and let it chill in the fridge for 30 minutes to soak up the flavors.
Then, arrange your tostada shells, layer on the avocado slices, spoon on the tuna mixture, sprinkle with furikake seasoning, and dig in right away for a delicious treat!