Take a medium sized bowl and combine 350g sushi-grade diced tuna fillets, 60g diced cucumber, 1 peeled, stone removed and diced avocado, 4 chopped spring onions, 1 1/2 teaspoons chilli flakes and 1 tablespoon toasted sesame seeds in it.
Then pour 2 teaspoons sesame oil, 1 1/2 teaspoons lemon juice and 125 ml soy sauce in this tuna mixture and carefully stir it until it is blended without mashing avocado.
After blending the ingredients place this bowl in a large bowl, which should be filled with ice. Then put this larger bowl in the refrigerator for about 15 minutes to chill and not longer than this.
When the tuna mixture is chilled, then take it out of the bowl of ice and transfer it into a serving plate, as it is ready to be consumed.