Mix the flour, water, and dashi powder in a large bowl. Add the eggs and stir until just combined — a few lumps are fine. Fold in the shredded cabbage, tenkasu, and beni shoga until evenly coated.
Heat a frying pan over medium heat with a little oil. Pour half the batter and shape into a round, flat pancake about 2cm thick. Lay 2–3 slices of pork belly on top.
Cook for 4–5 minutes until the bottom is golden and the edges look set. Slide a spatula underneath and flip the whole thing in one confident motion. Cook the other side for another 4–5 minutes.
Transfer to a plate. Layer on okonomiyaki sauce, then Japanese mayo in a zigzag pattern. Sprinkle generously with katsuobushi and aonori. Serve immediately.