I’d maybe refer you to Sandwich: A Global History by Bee Wilson. I’m going to try to summarise her research. In the first chapter, she discusses the frustrating lack of evidence of there being something like what we recognise as sandwich- that is a piece of meat or cheese or perhaps sliced tomato between two slices of bread. She notes that the Larousse gastronomique states that it was common for farm labourers to receive something between two slices of bread, but this kind of peasant meal must have been widespread across Europe. It’s just there was no common name except for “bread and cheese” perhaps. The tradition wasn’t well known among the wealthier classes in France because a French traveller credits the Earl of Sandwich with its invention. It’s unfortunately often the case that only the food of the wealthy is really documented and attested.

But while we know people have been eating bread and cheese or bread and meat for forever, there’s not very much evidence of it being between two slices of bread rather than served side by side on a plate. There’s no paintings of sandwiches, and while there’s plenty of written reference to bread and cheese (for example), there’s nothing that we can firmly say is a description of a sandwich.

Bee Wilson dismisses categorising the trencher as an early sandwich. This was where bread was essentially used as a plate to sop up any gravy, but often the bread was left uneaten. Not very much like a sandwich then. She suggests maybe something called a “spread toast” might be more like a sandwich, but it is maybe more an open sandwich with something on top, though allegedly the Earl if Sandwich’s sandwich was meat between two pieces of toast, so maybe there’s something in the spread toast. Another contender were travelling loaves, where a whole loaf would have its innards scooped and filled with meat or cheese or some other filling to take on the road with you.

Bee Wilson suggests the first sandwich proper that we have documented in from the 1st century AD, and is the Hillel sandwich eaten ritually at Passover by Jews. Here lamb and bitter herbs is sandwiched between two pieces of flatbread- and it would have been flatbread back then not the harder matzah cracker, although she points out that the name for this sandwich is Korech, deriving from a word meaning encircle, so Hillel’s sandwich might have been a wrap or a stuffed flatbread. She however feels a stuffed flatbread common in the Middle East isn’t really what most people consider a sandwich.

So that brings us back to the Earl of Sandwich. He may not have invented it, though there’s nothing firm to say otherwise, but he had the social position to make eating a sandwich a respectable thing for the wealthier classes, and leading to it being properly documented for the first time.