This is my version based on a combination of other recipes I've found.

Use amounts that seem right to you. It should be mostly about the tuna, with the other ingredients added for flavor:

Very fresh Ahi (yellowfin tuna) fillet -- cut in 1/2" cubes (you could use sushi-grade if you're worried about it)
Soy sauce
Sesame Oil (use only several drops since it can be really strong)
Sesame seeds (toasted in a heavy pan even better)
A little bit of chile sauce like Sriracha
Schichimi pepper (a spicy mixture of several peppers that you can find in Asian markets)
Salt (pink Hawaiian salt would be even better)
Arame seaweed (limu manauea if you can find it)
Green onions, chopped

Combine all ingredients in bowl. Toss gently. Let sit until seaweed is softened.

It's really good served with a salsa made of mango, pear, jalapeno a little oil and vinegar, lemon juice and salt & pepper.