As olive oil and other vegetable oils (e.g. sesame oil, which Jews had used in Mesopotamia) were unavailable in northwestern Europe, Ashkenazi Jews turned to animal sources, like their Gentile neighbors. However, Kashrut prohibited Jews from using the most common cooking fats in northern Europe, namely butter and lard. Butter, being derived from milk, cannot be used with meat under the Jewish prohibition on mixing meat and dairy, while lard is derived from pork, a meat not considered kosher. Furthermore, even among the less common fats available, tallow derived from beef or mutton would have been uneconomical, particularly given that virtually all suet (the raw material for tallow) is "chelev" and its consumption is forbidden. Thus Ashkenazi Jews turned to poultry fat as their cooking fat of choice. This fat, which they called schmaltz, became the most popular cooking fat used in the shtetls (Jewish villages) of central and eastern Europe. It was commonly used in a multitude of dishes served with, or containing, meat in accordance with kosher dietary laws. The practice of overfeeding geese to produce more fat per bird produced modern Europe's first "foie gras" as well.