Spread a sheet of plastic wrap on the counter, and place a few avocado cubes on it. Cover with another sheet of plastic wrap. Using a flat-bottomed pan, tap on the avocado cubes to flatten them into very thin slices. Place the slices in the freezer for 45 minutes or until ready to serve.
Slice the radishes into a fine julienne and mix them with the rice vinegar and sugar. Marinate for 15 minutes, then drain.
On a serving plate, carefully arrange the avocado slices and top with slices of tuna. Drizzle the orange juice over top.
Garnish with the rest of the ingredients, season and serve.
If you have no lemon olive oil, replace it with olive oil and fresh lemon zest.