Add the sausages: Nestle the sausages on top of the onions in a single layer if possible. They will sink down into the onions as everything cooks and softens.
Mix the simple gravy base: In a measuring cup or bowl, stir together the beef stock and Worcestershire sauce. Taste the mixture; it should be savory and a bit tangy. (You can season with a pinch of salt and pepper if you like, but to honor the 4-ingredient spirit, it’s not required.)
Pour over and cover: Pour the stock and Worcestershire mixture evenly over the sausages and onions. The liquid should come at least halfway up the sausages. Cover the slow cooker with the lid.
Slow cook until tender: Cook on LOW for 6–8 hours, or on HIGH for 3–4 hours, until the onions are very soft and translucent, the sausages are cooked through and tender, and the liquid has darkened into a rich brown onion gravy.
Finish the gravy texture: If you’d like a slightly thicker gravy, remove the lid for the last 30 minutes of cooking to let some liquid evaporate, or transfer a ladleful of onions and liquid to a bowl and mash them with a fork before stirring back in. The onions themselves are the thickener here, just as my grandfather did—no flour or cornstarch needed.
