Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine flour, dashi stock, eggs, soy sauce, mirin, and baking powder. Mix until the batter is smooth.
Add shredded cabbage, mixed seafood, green onions, pickled red ginger, and tenkasu to the batter. Gently fold in the ingredients until well combined.
Grease a baking dish with oil. Pour the okonomiyaki batter into the dish, spreading it evenly.
Bake in the preheated oven for 25-30 minutes or until the top is golden brown and the center is cooked through. You can test for doneness by inserting a toothpick into the center; it should come out clean.
Once baked, remove from the oven and let it cool for a few minutes.
Drizzle okonomiyaki sauce and Japanese mayonnaise over the top. Sprinkle bonito flakes and aonori for added flavor and presentation.
Slice the baked okonomiyaki into wedges and serve immediately.