Goat cheese and red onion is the goat cheese sandwich with sharpness set against sharpness, and the interest is in how the two sharps differ. The base is the familiar bright, tart, faintly chalky soft cheese, and the variable here is an onion that has been softened rather than left raw. Cooked slowly until it collapses, a red onion turns sweet, jammy, and faintly winey, its bite replaced by a dark, sticky depth. Set against the lactic tang of the cheese, it does not cancel the acidity so much as give it somewhere to go: the cheese stays clean and sour, the onion answers with caramelised sweetness, and the contrast holds because one is wet and yielding while the other is fresh and chalky. Raw red onion changes the sandwich into something else entirely, all pungent crunch, and the codified version is the slow-cooked one for a reason.
The craft is in the onion's water and the cheese's job. A red onion sweated down releases a great deal of liquid, so it is cooked until that liquid has gone and what remains is a thick, almost spreadable confit rather than a wet pile, because anything looser soaks the bread before it reaches the hand. The goat cheese is spread to the edges and acts, as it does in this whole family, as a fat-rich seal between a moist filling and the crumb. The bread is plain and soft so it does not compete with two assertive components, and butter underneath softens the cheese's edge and bridges its salt. A few toasted walnuts or a leaf of something peppery sometimes goes in, but sparingly: the onion is already doing the sweet-against-sharp work, and a third loud element muddies a balance that depends on being a duet.
This is the goat cheese family's allium member, the one whose variable is a softened, sweetened onion rather than a root or a fruit. Its near variants are set by how far the onion is taken: a light sweat keeps some bite, a long slow cook turns it to confit, a splash of vinegar at the end pushes it toward a pickle. The plain goat cheese sandwich and the beetroot version are separate builds. Those deserve their own articles rather than being crowded in here.