Cheese savoury is not a cheese sandwich with extras; it is a cheese sandwich whose filling has been taken apart and rebuilt into a single bound paste, and the bind is the entire point. Grated cheese, finely chopped or grated onion, mayonnaise or salad cream, and usually a shake of Worcestershire sauce are worked together in a bowl into a soft, spreadable, pale-orange mixture before any of it goes near bread. A plain cheese sandwich is a slab between slices; this is a homogeneous spread where the cheese, the onion sharpness, the creamy tang, and the savoury depth are distributed through every part of every bite. That conversion from layered to bound is what gives it its own name and its own behaviour.
The craft is the ratio in the bowl, because once it is mixed there is nothing left to adjust. Too little mayonnaise and the grated cheese stays loose and dry and falls out of the sandwich in a shower; too much and the mixture slumps into a wet slick that soaks the bread from the centre. The correct amount is just enough to make the cheese cohere into a paste that holds a shape when spread and cut. The onion is grated rather than sliced so it disappears into the body of the mixture and contributes sharpness everywhere rather than a bite in one place, and it is used in a measured quantity because raw onion left to sit only grows louder. The Worcestershire sauce is the savoury floor the name points at, added by the drop, enough to deepen the mixture without announcing itself. The bread is soft and plain and buttered, the butter sealing the crumb against a filling that is by design slightly wet, and the spread is taken to the edges so every bite is the same rather than mostly bread at the corners.
The variations are mostly arguments about the binder and the depth. Salad cream in place of mayonnaise pushes the whole mixture tangier and thinner; a sharper Worcestershire hand makes it darker and more savoury; some hands work in a little mustard or pickle for an extra acid edge. The plain cheese sandwich is the same ingredients refusing to be mixed at all. Each deserves its own article rather than being crowded in here.