Pancake batter:
Place the cabbage in a bowl and bruise it by gently squeezing it in your hands, which helps to soften it. Set aside until needed.
To make the pancake batter, combine the flour, dashi powder and celery salt in a bowl. Make a well in the center, then add the egg and gradually whisk in enough water (about 450 ml) to make a smooth batter the consistency of pouring cream. Pour into a jug.
Preheat a large flat barbecue hotplate or heavy-based non-stick frying pan over medium heat. When hot, add a drizzle of oil, then pour a thin layer of batter to make a 23 cm round pancake. Pile one-quarter of the cabbage and spring onions on top. Sprinkle with a little pickled ginger, then drizzle over a little more batter to help enclose the filling when turned. Cook for 5 minutes, then use a wide spatula to flip the pancake over and cook for another 3-5 minutes or until golden and crisp. While the pancake is cooking, heat another large frying pan over high heat. Drizzle in a little oil and when hot, cook the fish, flesh-side down, for 1-2 minutes or until nearly cooked through. Turn and cook for another 1-2 minutes or until the skin is crisp, then remove from the pan and set aside.
Transfer the pancake to a serving plate, top with 3 pieces cooked fish, then scatter with a little kimchi. Drizzle with okonomi sauce, then drizzle generously with mayonnaise. Cut into slices and serve immediately. Repeat with the remaining ingredients to make 4 pancakes in total.