The loco moco is a knife-and-fork sandwich that has given up the second slice and replaced the first one with rice. A mound of short-grain white rice is the base; a griddled hamburger patty sits on it; a fried egg goes on the patty; a ladle of brown gravy is poured over the whole thing. There is no bread and nothing to pick up. The rice is the structural plate you are also meant to eat, and the gravy is both the seasoning and the thing that binds the stack into one dish. The defining decision is that the carbohydrate base is loose, hot rice rather than toast, which changes how everything above it has to behave.
As an open-face plate it works because each layer is doing a specific structural job. The rice is packed dense and served hot so it can take a flood of gravy without dissolving into the meal too early, the same problem a Hot Brown solves with firm toast. The hamburger patty is cooked with a hard seared crust so it holds its shape and its juices under the weight of the egg and the sauce instead of breaking down into the rice. The fried egg is the binder: a soft, runny yolk broken into the gravy enriches it and threads the patty to the rice so a forkful comes up as one composed bite rather than three separate things. The gravy is the unifying element and is seasoned to carry the plate, since the rice brings none of its own. Timing matters the way it does for any open-face build: assembled too far ahead, the rice goes to paste before the fork reaches it.
The loco moco belongs to the Hawaiian plate-lunch tradition and sits in the open-face and hot-plate family alongside the Louisville Hot Brown, the Springfield horseshoe, and the diner hot turkey plate, all of which share the single-base, knife-and-fork, gravy-bound logic. Its own variations swap the protein, Portuguese sausage, kalua pork, teriyaki beef, or fried fish, while keeping the rice, egg, and gravy fixed, and the occasional version that stuffs the components into a bun pulls it back toward a handheld sandwich. Each of those deserves its own article rather than being crowded in here.