Heat a takoyaki pan over medium heat to 400 degrees Fahrenheit. Use a brush to generously coat the pan with oil, making sure to oil the holes and flat areas.
When the pan starts to smoke, pour the batter into the holes. Don't worry if it slightly overflows.
Add a few fish pieces to each hole and sprinkle the ground powder katsuobushi on top.
Sprinkle with tenkasu, pickled ginger, and green onion.
After 3 minutes, the bottom of the dumplings will slightly harden. Break the batter between each ball with a skewer, then turn each by 90 degrees and push in the edges as you turn it. The batter will flow out from the inside and create the other side of the ball.
Set a timer for 4 minutes and keep turning constantly.
Put the takoyaki balls onto a platter and serve with takoyaki sauce and Japanese mayonnaise. Sprinkle with katsuobushi and dried seaweed.
Serve immediately.