A coddie is a sandwich whose bread is two saltine crackers, and that constraint defines everything else. A cake of cod and mashed potato, bound and fried, is set between a pair of saltines with a stripe of yellow mustard, and the crackers are not a substitute for bread so much as a deliberately minimal frame: dry, salty, brittle, contributing almost nothing but a crisp snap and a place for the mustard to sit. The filling has to carry the entire sandwich because the bread, by design, refuses to. That is the engineering decision the whole thing rests on, and it is what makes a coddie a coddie rather than a fish cake on a plate.
The craft is in the cake and the bind. Salt cod or fresh cod is flaked and mixed with mashed potato in a ratio weighted toward the potato, which stretches a small amount of fish and, more importantly, gives the cake enough starch to hold together through frying and through being picked up by hand. Crackers, egg, and seasoning tighten the mix; the cake is fried until the outside is crisp and the inside stays soft and just bound. The saltine matters precisely because it is so plain: it adds salt and a dry crackle that cut the soft, rich, fried interior without competing with it, and it keeps the whole thing cheap, which was the entire point of the format for the dockworkers and corner stores it fed. The mustard is the only condiment, sharp and acidic, the single counter to a filling that is otherwise mild starch and fish. There is no lettuce, no sauce, no second decision. A coddie is made and held at room temperature at a corner store or a tavern bar, eaten in two or three bites standing up, and its restraint is not an accident but the recipe.
The variants are minimal because the form barely permits them. The fish-to-potato ratio shifts with who is making it, leaner toward the cod or cheaper toward the potato; some builds finish with a dash of hot sauce alongside the mustard. It belongs to the broad American fried fish sandwich family, and those richer, bun-based relatives deserve their own articles rather than being crowded in here.