Combine the red onion, lime juice, and vinegar. Let it sit while you slice your cucumber thinly. Toss it all together with a pinch each of salt and sugar. Let everything sit for a few minutes, so that the onion and cucumber soften and almost begin to pickle. Add the sesame oil and toss again.
Taste a cucumber; if you're happy with how things are going, toss in the shiso; otherwise, adjust the seasonings to taste before you add your herbs. Garnish with toasted sesame seeds; I like black ones, for a bit of contrast. Add the pepper flakes if you're jonesing for some heat. I served this with some sliced avocado (in a fan, natch) and brown rice. I called it dinner.