sulguni
Georgian brined cheese made from cow's milk and stretched like mozzarella, so it is elastic, layered in fibres, salty and mildly sour.
Sulguni is a cheese from western Georgia, usually made from cow's milk and sometimes with buffalo milk. The curd is stretched and kneaded in hot water and then stored in brine. It is firm and elastic and pulls apart into fibres and layers. It tastes noticeably saltier and sourer than mozzarella. When heated it melts softly and draws long strings without releasing much fat, which makes it the classic cheese for baked and filled breads. It comes fresh or smoked, and the smoked kind is firmer and more savoury. Look for it in Russian and Eastern European grocery shops, occasionally at well-stocked cheese counters, and from online retailers.
Substitute:
The closest match is a mix of low-moisture mozzarella and some crumbled feta, roughly two to one. The mozzarella gives the melt and the stretch, and the feta brings the salt and tang. Mozzarella on its own is too mild and watery, and feta on its own will not melt. Smoked scamorza can stand in for smoked sulguni, though it is a little less salty.