Avocado and egg is the hardest of the avocado sandwiches to get right because both halves of it are soft. Ripe avocado is smooth and fatty; a fried or poached egg is tender with a liquid centre. Put them together without thought and the result is a rich, slippery, single-textured filling with nothing to push against, the kind of sandwich that tastes of very little despite being made of two generous things. The defining problem is therefore textural, not flavoural, and the whole build is an answer to it: how do you stop two soft, mild components from cancelling each other out.
The craft is contrast and the management of the yolk. The egg is the structural variable. A set yolk makes a stable, tidy sandwich but forfeits the one moment of drama the pairing offers; a runny yolk gives a sauce that coats the avocado and binds the sandwich but turns it into a knife-and-fork affair that floods the hand. Either choice is legitimate, but it has to be made deliberately rather than discovered halfway through a bite. Because both elements are soft, something hard usually has to be added at the bread: a properly toasted slice, a scatter of seeds, a crack of pepper and flaked salt worked into the avocado so the filling is at least seasoned even if it cannot be made crunchy. The avocado is seasoned in its own right rather than waiting on the egg, and the two are layered with the egg uppermost so its heat is felt and its yolk, if loose, runs down through the avocado rather than straight out. It is a brunch sandwich, made and eaten in the same few minutes.
The variations mostly import the missing crunch or salt: a rasher of bacon, a handful of leaves, chilli for heat. Each tips the sandwich toward a named pairing of its own, and those deserve their own articles rather than being crowded in here.