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. Try 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 and take out immediately or the freshness of the fish will be lost.
Once chilled, remove the bowl from the ice and place onto a serving plate. Serve with toasted bread from our Bakery or your favorite crackers.