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Crack the egg into a large bowl and tip in the flour, bouillon powder and 100ml water. Mix well. Stir in the shredded cabbage and season well.
Heat the oil in a small, deep frying pan or skillet (ours was around 19cm) over a medium heat and fry the pork belly slices for 4-5 mins until golden and the fat has rendered. Pour the cabbage mixture over, stir everything together and fry for 5 mins until dark brown underneath and semi-cooked. Flip and cook for 5 mins more.
Meanwhile, make the sauce by whisking all the ingredients. Season lightly.
When the okonomiyaki is cooked, transfer to a serving plate and top with the sauce, mayo, green nori, spring onions and katsuobushi, if using.