suluguni cheese
Georgian brined cheese from cow's or buffalo milk, scalded and kneaded like mozzarella but firmer and saltier, with a gently sour note.
Suluguni comes from western Georgia and is made by scalding and stretching young curd in hot water before it matures in brine; it is sold both fresh and smoked. The cheese is elastic and stringy, pulls apart in layers, and tastes milky, distinctly salty and slightly sour. When heated it melts evenly and stretches without releasing much fat, which is why it is prized for baked filled breads and for pan-frying. You can find it in Russian and Eastern European grocery shops, Georgian stores and by mail order, and occasionally at the cheese counter of larger supermarkets.
Substitute:
The closest match is a mix of firm low-moisture mozzarella and some feta, roughly two to one: the mozzarella provides the melt and stretch, the feta the salt and tang. The result is softer and milder, and the feta crumbles rather than stretches, so a filling turns slightly grainy; for frying in slices the mix does not work, and halloumi is the better choice there, though it does not melt.