Slice the onion lengthwise into quarters, then very thinly slice each quarter lengthwise. You should have about 1 cup. Place in a sieve, add 1 teaspoon of the salt, and toss well. Set over a bowl and let stand for 10 minutes to drain.
Meanwhile, place the cabbage in a bowl and pour over boiling water to cover (about 4 cups). Let stand for a minute or two, then drain in a colander. Place back in the bowl and set aside.
Rinse the onion with cold water, then squeeze dry and add to the cabbage. Set aside.
Heat the 2 teaspoons sesame oil in a small wok or small skillet over medium heat. Add the ginger and cook for about 1 minute, stirring frequently to prevent sticking. Add the vinegar, and once it bubbles, pour the mixture over the salad. Toss to blend, then add the remaining teaspoon salt and toss again. The salad can be served immediately or left to stand for up to an hour so flavors can blend.
Just before serving, taste and add a little more sesame oil if you want to bring that flavor forward, as well as more salt if you wish. Add the coriander leaves and toss.