To firm up, place the tuna in the freezer for 15 to 20 minutes. Dicing will be simpler as a result.
Combine the onion, sesame oil, olive oil, salt, and cayenne in a medium-sized mixing bowl. Put aside.
Place the tuna on a chopping board and let it cool before slicing. Cut thin slices with the grain using a slicer or a sharp chef's knife. After that, cut the slices lengthwise. After flipping the tuna, cut it into tiny 1/8-inch dice. Fill the mixing basin with tuna. Toss the ingredients gently to combine. Taste and adjust the seasoning.
To serve, line the bottom of a serving bowl with slices of cucumber, top with a heaping dollop of tuna tartare, chopped avocado, seaweed, and a final pinch of salt.