Avocado toast is barely a sandwich and is honest about it: a single slice of bread, no top, the open face the entire format. What defines it is the contrast between a firm toasted base and a soft mashed topping, which is the same logic every open-faced sandwich runs on, scaled down to two real ingredients. The bread is the structural decision, not the avocado. It has to be toasted firm enough to carry a wet, heavy mash without going limp under it, which is why the California build leans on a sturdy crusted loaf rather than soft sandwich bread.
The craft is in the toast and the texture of the mash. The bread is cut thick and toasted until the surface is rigid and the interior still has some give, because a flimsy base collapses the moment the avocado goes on and the whole thing becomes a fork exercise. The avocado is mashed coarse rather than smooth, ripe enough to spread but left with some structure so it reads as food and not as paste, then seasoned simply: salt, acid from lemon or lime, sometimes chile flake or olive oil. The seasoning is the real work, because there is nowhere to hide a bland mash on a bare slice of bread. Timing matters less than on a hot open-faced plate, but it still matters: dressed too early the toast softens from underneath before it reaches the table.
The variations are additions onto the same single-slice frame rather than different sandwiches. A poached or fried egg is the most common, turning it toward a full plate. Smoked salmon, radish, tomato, feta, or a chile crisp each push it in a direction without changing the structure. It sits in the same open-faced, knife-and-fork family as the diner hot turkey plate and the regional hot-plate builds, sharing the rule that the bread is a base you are also allowed to eat. Those relatives each deserve their own article rather than being crowded in here.