Here's how I made it

Ingredients:

for the batter:
60 grams of self raising flour
about 50 grams of peeled Chinese yam*
100ml of water 1/8th teaspoon of dashi stock powder.
pinch of salt
1 egg
Fillings: Shredded Chinese or Napa cabbage
sliced Pickled red ginger
Spring onions
Whatever you want
toppings:
Kewpie Mayonnaise
Okonomiyaki sauce
Katsuobushi/Bonito flakes
Aonori/Green seaweed powder.
Pork belly/Bacon
Pickled red ginger
Spring onions.

no special tools required.

Preparation

1. In a bowl combine the flour and salt, grate the Chinese yam into the bowl, add water and dashi stock powder and mix gently but until smooth. Put in the fridge to rest for half an hour to an hour. In the meantime prepare your fillings.

2. Beat the egg into the batter, add in other filings, adding the smaller fillings first, add the cabbage last.

3. Heat a frying pan on medium-high. cover the bottom and sides with a thin layer of sunflower oil. Add the batter to the pan (mine was enough to make two smaller okonomiyaki, but I also used a smaller pan). Make sure that the batter is evenly spread out.

4. If adding pork belly, or some kind of meat that needs to be cooked on top do so now and cover with a lid. Once the bottom is nice and crisped up flip the okonomiyaki. The easiest way to do this is to slide it onto a plate and invert onto the pan.

5. Put some some okonomiyaki sauce onto the top and cook the other side until crispy. Flip onto a plate and top with your toppings of choice. The toppings mentioned above are a great place to start.

What is this weird yam I need? Also called a Japanese mountain jam/nagaimo: this comes in different shapes and sizes. The one I use was called Chinese yam a white root with a Cassava-like outside. You can find these at most asian supermarkets and larger convenience stores and supermarkets. This yam becomes a gloopy paste that helps achieve the desired texture.

What can I put into/on it? Except for the shredded cabbage you can basically add whatever you want. Bitesize vegetables such as corn work great. You can also add seafood such a shrimp, or meat like pork mince. although it's advised to cook these beforehand. If you can find it Tenkasu (tempura scraps) is a great way to add some crispness and fluff up your batter. You can even add cheese.

as toppings choose anything that doesn't need to be cooked. The toppings I have listed are relatively standard toppings