Here's a basic recipe for starters. I lived on Oahu for 14 years and tried many variations of this dish. I love eating this stuff. I hope this gives you a starting point.
4 cups ahi (yellowfin tuna, sashimi grade) diced
1/2 cup onion, minced
1/4 cup green onion, minced
1 cup soy sauce
2 tablespoons sesame oil
Salt and pepper to taste
Season ahi with Hawaiian salt or sea salt. Add all ingredients, mix well then chill.