Slice the tuna against the grain, rotate, then slice it the other way, until you have evenly-chopped cube-like pieces. Add to a mixing bowl.
Cut the cucumber the same way so that the pieces are roughly the same size as the tuna. Add to the same mixing bowl.
Grate the onion and ginger into another mixing bowl. Mince the chilis and add them. Then add the soy sauce, olive oil, sesame oil, and rice vinegar. Add the sugar and whisk to combine. Toast the sesame seeds by putting them in a dry pan for a few minutes until they give off a toasty aroma. Then add them to the marinade. Roughly chop the mint and set aside until ready to serve.
When ready to serve, add the tuna and cucumber to the marinade. Toss several times. Add the mint and toss again. Serve immediately.