Bring rice, quinoa, barley and the water to the boil in a medium saucepan. Reduce heat; simmer, covered, for 30 minutes or until water is absorbed. Remove from heat; stand, covered, for 10 minutes.

Meanwhile, combine vinegar, sugar and salt in a small bowl. Place rice mixture and seeds in a large, wide, stainless steel bowl. Using a plastic spatula, repeatedly slice through rice mixture at an angle to separate grains, gradually pouring in vinegar mixture. Stand for 10 minutes to cool.

Grease and line a 19cm square cake pan with plastic wrap. Place one nori sheet, trimming to fit, shiny side-down, over base of pan. Spread half the rice mixture over nori. Top with salmon; sprinkle with chilli.

Combine avocado and juice in a small bowl. Place avocado mixture over salmon; top with remaining rice mixture. Place remaining nori sheet, trimming to fit, shiny side-up, over rice mixture. Cover nori with plastic wrap. Place food cans on sushi to weight it down. Refrigerate for 6 hours or until firm enough to cut.

To make pickled cucumber, combine ingredients in a medium bowl; stand for 5 minutes for flavours to develop. Drain.

Remove sushi from pan; cut into squares. Serve with pickled cucumber.