skyr
Icelandic cultured dairy made from skim milk and strained until thick and creamy, with a mild tang.
Many supermarkets stock skyr next to yogurt and quark, but smaller shops often don't carry it, and flavoured versions are no use in cooking. Recipes need the plain, unsweetened kind.
Substitute:
Low-fat quark stirred smooth with a little plain yogurt comes closest in texture and tang. Strained Greek yogurt also works but tastes richer and less sour.