In a large bowl, mix the flour, salt, and baking powder.
Peel and grate the nagaimo. Add the grated nagaimo and dashi into the bowl, and mix to combine.
Core and roughly chop the cabbage.
Add the cabbage and the rest of the batter ingredients (expect the optional pork belly) into the bowl and mix.
Heat oil in a large skillet over medium heat. Add some batter into the pan and shape it into a circle.
Place a few slices of pork belly on top, if using, and add some more batter on top to help the pork to stick.
Cook the pancake until browned on the bottom. Flip and cook until the other side is also browned.
Mix all sauce ingredients together.
Add the okonomi sauce and your desired toppings to the okonomiyaki and serve.