Tuna cut into very small cubes;
Chop the onion and cilantro in a blender to fine crumb;
Add chopped vegetables, lime juice, sesame oil and soy sauce to tuna.
Spread the resulting mass on a plate in a special salad form (if there is no form, you can use a small saucer with low sides), tamp with a spoon.
Remove the form (or turn the saucer on a plate for serving).
From above decorate sesame seeds.