Cut tuna into ¼ inch cubes and place in a large bowl. Putting the fresh tuna in the freezer for an hour (until just firm) before cutting makes it easier to cut into even cubes.
In a smaller bowl, combine the olive oil, lime zest, lime juice, wasabi, coconut aminos, sriracha, salt, and pepper.
Pour this mixture over the tuna, add the scallions, and stir well to coat all the pieces.
Cut the avocado into ¼ inch dice.
Gently mix the avocado with the tuna, so that the lime juice covers them completely (this prevents browning).
Cut the cucumber into small cubes, and add to the tuna mixture. Mix well.
Add the toasted sesame seeds, and season to taste.
Chill in the refrigerator until ready to serve, and enjoy with cucumber rounds.
This tuna tartare can be served in a half avocado as the photo above, as a "dip" served on cucumbers rounds or in a bed of lettuce as a salad.