The Sandwich Spätzle is a carb-on-carb curiosity, and that is precisely what makes it worth a look. Spätzle are the soft Alsatian egg dumplings, a loose dough of flour, egg, and a little liquid pushed into boiling water so it sets into irregular short noodles, then drained and usually pan-finished in butter until the edges catch. Putting them inside a split loaf is the same impulse that produces a chip butty or a potato roll: a starch already cooked and seasoned, packed into bread because bread is what you eat it with. The build is a sturdy loaf, a generous spoon of buttered Spätzle, and not much arguing with it.
The craft is in managing softness against softness, which is the obvious objection and also the whole technique. Spätzle on their own are tender, mild, and a little slippery, so a sandwich of them needs two things the plain dumpling does not: a bread with real structure to provide the contrast the filling cannot, and a sharp or savory accent so the whole thing is not one flat note of butter and wheat. The Alsatian kitchen tends to supply that accent from its own shelf: browned onions, a handful of grated melting cheese stirred through while the Spätzle are still hot, a few lardons, a spoon of the pan butter carried into the bread. The dumplings should be pan-finished rather than just boiled, because the crisped edges are the only textural relief the filling brings. It is best a few minutes from the pan, while the cheese is still molten and the bread still firm, since both the dumpling and the loaf go slack as they cool.
Variations stay inside the Alsatian register. A Spätzle au fromage build leans hard on the melted cheese and browned onion and reads like a portable gratin. A version with lardons or a slice of smoked sausage adds the salt and chew the dumpling lacks. Some cooks fold in a little crème fraîche so the filling binds into a cohesive layer rather than spilling. The frame is constant: cooked egg dumplings, a savory or salty lift, a bread sturdy enough to hold them. It belongs with the regional dishes folded into bread that the catalog groups under Plat-en-Sandwich, and its specific contribution is the frank starch-in-starch experiment, a cooked Alsatian dumpling treated as a sandwich filling and made to work by the bread and the seasoning around it.