The slider is defined by a cooking method that is the opposite of every griddle burger: it is steamed, not seared. A small, thin square patty is set on a bed of shredded onion and cooked through with steam rather than driven hard against a hot flat-top for a crust. That single decision is the whole sandwich. There is no Maillard lacquer and no charred edge, because the steam keeps the patty pale and soft and pushes onion flavor and moisture up into the meat and the bun together, so the result is uniformly tender rather than crusted and chewy.
The craft is in the bed of onion and the bun. The shredded onion is not a topping laid on after; it is the cooking surface, spread on the steam table so the patty sits in it and the rising vapor carries its sweetness and steam through holes in the patty into the bread above. The bun is small and very soft, sized exactly to the little square patty, and it is set over the cooking patty so it steams along with it rather than being toasted separately. A single pickle slice is the only acidic and firm element, deliberately spare, because the sandwich is small and one sharp note is enough to cut a soft, oniony, mild build. Everything is scaled down on purpose: the patty is thin and square so it cooks fast in steam and matches the small bun, and the whole thing is light enough to eat several in one sitting, which is the structural logic of selling them by the bagful.
The variations stay inside the small, steamed frame. A slice of cheese melted on under the bun, or a different patty filling, changes the flavor without changing the steam-and-onion method that defines it. Scaling the patty up or searing it on a flat-top turns it into a different, crusted burger and abandons the entire logic of the steam bed. Those builds deserve their own articles rather than being crowded in here.