Place the tuna in the freezer for 15-20 minutes to firm. This will make it easier to dice.
In a medium mixing bowl, add the chives, onion, sesame oil, olive oil, salt and cayenne. Set aside.
When the tuna is ready to be sliced, place on a cutting board. Using a slicer (or a sharp chef’s knife), make thin slices with the grain. Next, take the slices and slice lengthwise. Turn the tuna and cut into small 1/8-inch dice. Place tuna in the mixing bowl.
Gently toss the ingredients to incorporate. Adjust the seasoning to taste.
To serve, place cucumber slices on the bottom of a serving bowl, add a generous dollop of tuna tartare, topping with diced avocado, seaweed and a dash of finishing salt .