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1
Combine cabbage, scallion whites and half of greens, half of beni-shoga, 3/4 of katsuobushi, yamaimo, eggs, and water (or dashi) in a large bowl. Sprinkle with flour. Stir with a fork and beat heavily until a thick batter with plenty of bubbles forms. Set aside.
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2
If using pork belly, cover the bottom of a 10-inch nonstick skillet with pork belly and set over medium heat. Add okonomiyaki mixture and spread into an even layer with a fork. If not using pork belly, heat vegetable oil in skillet over medium heat until shimmering. Add okonomiyaki mixture and spread into an even layer with a fork.
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3
Cover and cook, shaking pan occasionally, until bottom layer is crisp and well browned, about 10 minutes, lowering heat as necessary if cabbage threatens to burn.
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4
To flip, drain off any excess fat, then, working over a sink and holding the lid tightly against pan with a pot holder, flip entire pan and lid over so that okonomiyaki transfers to pan lid. Remove pan, then carefully slide okonomiyaki off lid and back into pan, browned side up.
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5
Return to heat, cover, and continue to cook, shaking gently, until both sides are browned and okonomiyaki is not runny but still custardy and tender in the center, about 8 minutes longer. Transfer to a serving platter, pork side up.
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6
Drizzle with okonomiyaki sauce and mayonnaise; sprinkle with ao-nori, remaining beni-shoga, remaining katsuobushi, and remaining scallion greens; and serve immediately off of a communal plate.
