Cut the tuna into 1/3-inch cubes.
In a medium mixing bowl, add diced tuna, toasted sesame seeds, green onions, lemon zest, and soy sauce. Mix well. Taste one piece of tuna and add more seasonings if needed.
Cut the avocado in half and remove the pit.
Scoop into a mixing bowl. Mash the avocado with a fork until it’s slightly chunky.
Add salt, pepper, lemon juice, optional jalapeno, and cilantro. Mix until combined.
Taste test and add more salt if needed.
Place a food ring mold (you can make one at home with a plastic bottle) on a plate.
Add the avocado mixture at the bottom. Then add the tuna mixture on top. Gently pressing down to make sure there are no gaps. (You can also use a ramekin to shape your tartare. You’ll need to fill it with the tuna mixture and avocado mix. Then reverse it into the serving plate.)
Lift and remove the mold gently. Garnish with more sesame seeds and cilantro. Serve the tuna tartare immediately.