Olive oil was among the ubiquitous ingredients around the Mediterranean, but remained an expensive import in the north where oil of poppy, walnut, hazel and filbert was the most affordable alternative. Butter and lard, especially after the terrible blood-letting of the population during the Black Death, was used in considerable quantities in the northern and northwestern regions, especially in the Low Countries. Almost universal in middle and upper class cooking all over Europe was the almond, which was in the ubiquitous and highly versatile almond milk, which was used as a substitute in dishes that otherwise required eggs or milk, though the bitter variety came along much later.