Okonomiyaki restaurants (okonomiyaki-ya) serve large pancakes made with diced seafood, meat, cabbage and other vegetables. Okonomiyaki roughly means "cook what you like," and customers get to choose their own favorite ingredients and sometimes even cook the pancakes themselves at the table.
There are two main styles of preparation, identified with the cities of Osaka and Hiroshima, and each style has its enthusiastic proponents. In the Osaka version, which is more common, all the ingredients are mixed together in batter and everything is cooked like a pancake on a hot grill, flipped over halfway through so that both sides cook evenly.
Hiroshima-style okonomiyaki (also called Hiroshima-yaki) is built up in alternating layers of batter and other ingredients, including noodles (usually yakisoba) and more cabbage. It takes longer to assemble, and is prepared by the restaurant's chef, not the customer. There is another variation of okonomiyaki called monjayaki, which is a somewhat thinner, more watery pancake. This is generally found only in Tokyo, particularly in the Tokyo Bay neighborhood of Tsukishima, which is home to several dozen monjayaki restaurants.
The menu of an okonomiyaki restaurant will list the various main filler ingredients available, such as pork or shrimp. An order of okonomiyaki consists of a bowl of pancake-like batter and a dish containing diced vegetables and the meat or seafood.
The waiter or waitress will come by to turn on the grill at your table and brush the surface with oil; after that - if it's a cook-it-yourself place - the adventure begins. Start by mixing together all the ingredients; then, when the grill is hot enough, pour the mixture onto the grill. You'll find small spatulas with which to flatten the pancake and push it into shape, and a larger spatula with which to turn it over when the time comes. Before and after turning, brush the top of the pancake with Worcestershire-flavored sauce, and then sprinkle it with aonori (green seaweed powder) and katsuo (dried bonito shavings) before eating it.
It takes a bit of experience to figure out when to flip the pancake and when to take it off the grill. Okonomiyaki takes longer to cook than you might expect, and monjayaki takes even longer; in either case the finished product doesn't hold together nearly as well as a Western-style pancake. You might ask your waiter for advice or help, or else pay close attention to the technique of the people at the next table.