Combine the cabbage, scallion whites and half of greens, the fresh ginger, eggs, and water in a large bowl. Sprinkle with flour. Stir with a fork and beat heavily until a thick batter with plenty of bubbles forms. It might seem like too much cabbage, but don't worry!

If using lap cheong, scatter the lap cheong evenly in a 10" nonstick skillet and set over medium-low heat. Pan-fry on one side until crispy and dark golden. At this point, you can drain off some of the fat, but it's optional. Flip the lap cheong (no need to cook on this side before adding the okonomiyaki mixture).

If not using lap cheong, heat some oil in the skillet instead, then proceed.

Add the okonomiyaki mixture and spread into an even layer.

Cover and cook, shaking the pan occasionally, until bottom layer is crisp and well browned, about 10 minutes, lowering heat as necessary if cabbage threatens to burn.

To flip: working over a sink and holding the lid tightly against pan with a pot holder, flip the entire pan and lid over so that the okonomiyaki transfers to pan lid. Remove pan, then slide okonomiyaki off the lid and back into the pan, browned side up. I find that using a big plate is easier to do this with than a lid if the inside of your lid has ridges. Also: this might be scary, but don’t panic!! If it falls apart, smush it back together once it’s back in the pan and you’re all good.

Return to heat, cover, and cook, shaking gently, until both sides are browned and okonomiyaki is not runny but still custardy and tender in the center, about 8 minutes. Transfer to a serving platter, pork side up.

Drizzle with okonomiyaki sauce and mayonnaise. Sprinkle with ao-nori, pickled ginger and remaining scallion greens; and serve immediately off of a communal plate.