Combine red onion, green onions, ginger juice, soy sauce, rice wine vinegar, mirin, sesame seeds, lemon juice, and sesame oil (reserving some for later) in a non-reactive bowl.

Mix well and taste. Adjust seasoning, adding salt if needed.

Adjust the amount of sesame oil and red chili sauce to your preference.

Cut the fresh ahi tuna into small, uniform pieces, approximately 1/2 inch square.

Gently toss the tuna in the prepared dressing.

Refrigerate the mixture for 30 minutes to allow the flavors to meld.

Serve as an appetizer with shrimp chips or fried wonton chips.

Alternatively, serve as an entree with steamed short-grain rice.

Or, serve as a salad over seaweed or lettuce.