Combine the ingredients for the avocado layer in a bowl.
Combine the ingredients for the tuna layer in a seperate bowl. Taste to check if more seasoning is needed. But make sure the tuna mixture doesnt get too wet.
Assembly the tuna tartare by placing half of the avocado mixture at the bottom of a plate using a round mold (see note 1) making sure it gets a circulare form and stays in place. Gently push the avocado down to make sure it sit. Place the rest of the avocado in a seperate mold on a seperate plate.
Next divide the tuna layer between the two molds, also pressing it down gently to make sure it stays in place.
Top the tartare with diced cherry tomatoes and fresh basil.