Put water in a pot and bring it to a boil. Once boiling, reduce the heat to low, add bonito flakes, and let it simmer for 3 minutes.
Turn off the heat and strain the mixture through a fine mesh sieve into a bowl or measuring cup. The bonito dashi for okonomiyaki is ready. Mix in salt and let the mixture sit until it cools, for about 30 minutes.
In a small bowl, mix Japanese Worcestershire sauce, ketchup, and sugar. Loosely cover the bowl with plastic wrap and microwave it on medium power (500W) for 30 seconds. Okonomiyaki sauce is ready.
Coarsely chop cabbage. Finely chop beni shoga. Cut pork belly into 4-inch (10cm) lengths.
In a large bowl, lightly combine flour and baking powder with the cooled dashi (⅔ cup/160 ml per 2 servings) until the powder disappears. Divide the combined batter evenly into small bowls, one for each serving. On top of the batter in each bowl, place the chopped cabbage, beni shoga, tenkasu, and a cracked egg.Important note: Do not mix the contents of each bowl at this stage. (Cook one serving per pan unless you have a large griddle.) Heat oil in a pan over medium heat. Once the pan is hot, quickly mix the contents of one serving bowl until the egg yolk is lightly and uniformly blended. Place the mixture in the pan and shape it into a circular form about ⅘-inch (2cm) high using a spatula, then place the pork on top without overlapping as much as possible, and cook for about 5 minutes.
Once the underside of the mixture is crisply browned, carefully flip it over using a spatula and cook the pork side for about 5 minutes.
Once the pork side is crisply browned, carefully flip it over again. Continue cooking for about a minute to absorb the pork fat.
Serve on a plate, spread with the okonomiyaki sauce, and top with Japanese mayonnaise, aonori, and bonito flakes to taste. Continue to cook the next okonomiyaki.