quark
Fresh cheese made from soured milk; white, creamy to slightly crumbly, mildly tangy.
Quark is sold in every supermarket in German-speaking countries, but in the English-speaking world you mostly find it in larger supermarkets, Eastern European or Polish shops, and health food stores.
Substitute:
Farmer's cheese or well-drained cottage cheese pressed through a sieve come closest; strained Greek yogurt also works but is tangier and less firm.