The fried egg sandwich is the plain sliced-bread reading of the fried egg, and its defining problem is yolk control rather than yolk spill. Where the affectionate butty wants the yolk to break and run down the wrist, the sandwich is the version made on ordinary buttered slices and meant to be eaten more or less cleanly, which means the yolk has to be managed rather than indulged. The runny yolk is still preferred, the soft centre being the only sauce a two-ingredient sandwich has, but the whole build is arranged so that sauce stays roughly where it is put. The bread is the constant, plain sliced white or brown, and the craft is the discipline of keeping a liquid yolk inside a flat sandwich for the length of time it takes to eat one.
The craft is the fry and the seal. The egg is fried until the white is firmly set, because a slack white slides and a set one gives the yolk a wall to sit against between the slices. The yolk is left soft but the egg is laid yolk-up and the second slice pressed down gently and evenly, so the yolk is held under tension across the whole face rather than driven out one side by an uneven push. Butter spread to the edges is structural, a thin fat film that both carries the seasoning across bland bread and slows the slice from wicking up the trace of yolk and oil that does escape. Salt and pepper go directly onto the egg, since there is nothing else in the build to season it. Cut once across the middle, the sandwich is eaten with the cut edge tipped up so a broken yolk runs back in rather than out.
The variations are the act of adding back what the bare sandwich leaves out, or of letting the yolk go. The runny-yolk spill, embraced rather than contained, is the butty register entirely; a fully cooked yolk turns it into a firm, portable lunch-tin sandwich with no sauce at all; brown sauce or a rasher of bacon pushes it toward the breakfast reading. Each tips the sandwich toward a named build with its own logic, and those deserve their own articles rather than being crowded in here.