The key part of the definition is "placed on or between slices of bread,"
A hot dog bun is a single entity that is opened up to put the hot dog in
I'm glad to be able to resolve such a idiotic first world problem...donations accepted..
A sandwich is a food item consisting of one or more types of food, such as sliced cheese or meat, placed on or between slices of bread, or more generally any dish wherein two or more pieces of bread serve as a container or wrapper for some other food.[1][2][3] The sandwich was originally a portable food item or finger food which began to be popular in the Western World. Today sandwiches in various versions are found worldwide.
From Wikipedia, the free encyclopedia
An open sandwich, also known as an open face/faced sandwich, bread baser, or tartine[1] consists of a single slice of bread with one or more food items on top.