We love to make this for brunch. We top it with bacon and make the sauce from scratch (it's very easy). The only thing you specifically need to buy is the okonomiyaki flour and bonito flakes, the rest you can do with common ingredients. And don't forget to use japanese mayo, not regular mayo.
The result looks impressive for a first time! I find it hard to get the right balance of tastes.
I think my favourite one is in a little restaurant in Gion, Kyōto, where it is prepared in front of me on the teppan (鉄板) by the chef before being enjoyed piece by piece. You really eat it with all your senses, I love it.