Take Hiroshima-style okonomiyaki, the layered kind built on a thin crepe with a heap of cabbage, a tangle of yakisoba noodles, pork, and egg stacked and pressed on a griddle, then put the whole stack between bread. That is the Hiroshima okonomiyaki sando: a complete griddle dish wedged into a roll, regional fusion at its most literal and its most structurally ambitious.
The difficulty is that Hiroshima okonomiyaki is tall, layered, and sauced, none of which wants to live in bread. The good versions adapt rather than just transfer. The cabbage is cooked down so it loses volume and water, the noodle layer is kept thin and a little crisp at the edges so it acts as structure rather than slipping mass, and the pancake is compressed firmly so the strata bond into a single sliceable block before it ever meets the bread. A soft roll or milk loaf is typical, the inner faces toasted or buttered to resist the okonomiyaki sauce, which is sweet, thick, and very willing to soak. The bind you are looking for is a dense, cohesive okonomiyaki block that holds its layers when bitten and a roll that compresses with it; the failure mode is a roll fighting a loose, sauce-heavy stack that shears apart on the first bite and slides out the back, noodles trailing. Aonori, bonito, and a lattice of kewpie mayo usually finish it, and the mayo doubles as a little extra glue.
Eating it is a layered experience by design: the brief give of bread, then noodle chew, then sweet cabbage, then the savory pork and egg, with the dark sauce tying every stratum together and the seaweed and bonito adding a low, smoky background. The cabbage's faint sweetness and the sauce's heavier sweetness can stack up, so a sharp hit of mayo or a little pickled ginger does a lot to keep it from going one-note across a whole sandwich.
Variations track what goes into the stack. A noodle-forward build leans crisp and substantial; a cabbage-forward one eats lighter and sweeter. Cheese is a common addition that melts down into the noodle layer and binds it further. Some shops add extra pork or squid in the Hiroshima manner and lean fully into the regional reference. There is also an Osaka-style okonomiyaki sando, mixed rather than layered and noodle-free, which behaves quite differently in bread and deserves its own article rather than being crowded in here.