Koulouri me Zampon kai Tyri is the most sandwich-like member of the koulouri family: the sesame ring split and filled with ham and cheese. Where the plain ring is a one-hand breakfast and the cheese-only version is a light filled snack, this is a proper portable meal, the koulouri doing the job a roll or a length of bread would do elsewhere. The interest is in how a ring-shaped bread handles a two-component filling, because the geometry makes it harder than a straight loaf.
The build starts with a fresh koulouri, cut around its circumference and opened into two flat-ish rings, like splitting a thin bagel. Ham goes in, usually a sliced cooked zampon, and cheese with it, often a salty white or a meltable yellow. The fillings have to be laid around the ring rather than stacked, because the hole means there is no center to pile into. Good execution keeps the layers continuous all the way around the loop so every bite has both ham and cheese, uses a ring with enough crumb to anchor the filling without crushing, and keeps the proportions honest, enough ham to taste against the sesame, enough cheese to bind the bite. Sloppy work is a ring split so roughly it tears into pieces, filling bunched at one point so half the ring is bare bread, a stale crumbly koulouri that cannot survive being opened, or so thin a layer of ham that the salty sesame crust simply overwhelms it. The thing that makes this work is the contrast of roasted nutty crust, salty cured ham, and cool cheese, and it fails when any one of those is stale, mean, or unevenly placed.
Variation tracks the cheese and the ham. A brined white cheese leans tangy and soft against the ham; a firmer yellow cheese gives a drier, more structured bite that holds in the split. Some versions warm the filled ring slightly so the cheese softens against the ham, others serve it cold straight off the cart. The cheese-only form and the sweet chocolate form are built on the same split ring and each deserves its own article rather than being crowded in here, and the base sesame ring itself, the plain Koulouri Thessalonikis, is covered separately as the foundation this rests on.