Okonomi-yaki
Just try this. That is about all When Japan needs to say. It is a unique Japanese dish that you rarely find outside of Japan, yet is a distinct part of the culture and identity of Japan, especially in the Osaka and Hiroshima areas. The two main styles differ greatly and both should be sampled while in Japan. It is also one of the few dishes that should also be sampled locally if you manage to get away from Tokyo.
Do note however that in Tokyo it is next to impossible to find the Hiroshima style, so unless you actually go to Hiroshima, or occasionally in Osaka, you will be out of luck. In Tokyo, you can make the Osaka style Okonomiyaki yourself (a recommendation is Tsukishima) however in Osaka the staff will always make it for you and then bring it over to your table hot plate (making the reason for having a hot plate unclear – although it is used to make other dishes also).

Making Osaka style Okonomiyaki yourself
There are plenty of instruction videos online however they are a bit waffly and don’t focus on the key part, decorating the top with a good balance of ingredients. Mixing and grilling is the easy part; just don’t let it overcook (should be nicely brown before flipping over). The hard part is getting a good spread of okonomi-yaki sauce as a base, then topping that is an even criss-cross of mayonnaise, dashed with tuna flakes and scattered seaweed. There is a certain method to decoration, the mayonnaise especially should be liberally applied as the main bulk can get a bit dry. The sauce and mayonnaise help balance that out.
A recommendation in Osaka is Meijiro-tei, which is near Sakai station. There is no English menu but I’m sure you’ll figure it out. They also do the Hiroshima style dish. In Hiroshima, try Hassho.

Hiroshima Style 