Place the chopped jalapeños and the brine in a medium bowl. Add the onion, toss to coat in the brine, and leave to quick-pickle for 10 minutes.
Lightly drain the tuna, leaving it still a little oil-slicked. Add the tuna to the bowl and flake it with a fork. Add the avocado and cilantro, if using, and toss to combine, mashing some of the avocado lightly while leaving some diced. Taste and season with salt as needed.
