In a medium bowl, combine the tuna, cucumber, avocado, green onion, red pepper flakes and sesame seeds. Pour in the lemon juice, sesame oil and soy sauce, and stir carefully to blend so as not to mash the avocado. Place this bowl into a larger bowl that has been filled with ice. Chill in the refrigerator for 15 minutes, but no longer - the terrific freshness of the fish will be lost.
Once chilled, remove the bowl from the ice, and invert onto a serving plate. Serve with toasted bread or your favorite crackers.