1/2 small head cabbage, finely shredded (about 4 packed cups; 14 ounces; 400g)
3 scallions, thinly sliced, dark green parts reserved separately
2 ounces (50g) beni-shoga (Japanese pickled red ginger), divided (see note above)
1/2 ounce (15g) katsuobushi, divided (see note above)
1/4 pound (120g) yamaimo, peeled and grated on the smallest holes of a box grater (optional; see note above)
2 large eggs
1/2 cup (120ml) cold water or dashi (or use cold water with 2 teaspoons Hondashi; see note above)
3/4 cup all-purpose flour (3 3/4 ounces; 110g)
8 to 10 thin slices uncured pork belly (optional; see note above)
2 tablespoons (30ml) vegetable oil (if not using pork belly)
Ao-nori, okonomiyaki sauce, and Kewpie mayonnaise, for serving (see note above)