In a mixing bowl, whisk together plain flour, baking powder, stock, and egg until no lumps remain. Add grated potato and black pepper and mix well. Cover and let the batter rest for at least 30 minutes.
While the batter rests, cut the cabbage into rough pieces about 3 cm across, not finely shredded, and dry it well in a salad spinner or a clean tea towel. Keep it in a separate bowl until you are ready to cook.
Mix the okonomiyaki sauce ingredients together, cut the bacon in half, slice the spring onion and shallot, and grate the ginger and garlic.
Fold the cabbage, prawns, ginger, garlic, shallot and spring onion through the rested batter for about 30 seconds. Do not beat it. If the mixture looks loose, beat in another egg.
Heat a frying pan over a medium heat with 1 tbsp of oil. Add a quarter of the mixture and shape it into a round about 2 cm thick, then lay half the bacon on top. Do not press it down: Osaka style is meant to stay tall and airy, and flattening it forces out the air.
Cover the pan and cook for 5 to 6 minutes, until the edges look set and the base is deep golden.
Flip it in one decisive movement, using two fish slices if that helps, then cover again and cook for another 5 to 6 minutes. It is ready when the prawns are pink and opaque and a skewer pushed into the thickest part comes out clean and hot.
When ready, remove the Okonomiyaki from the pan and garnish it with sauce, mayonnaise, and katsuobushi. Repeat the process with the remaining batter. Serve hot and enjoy!