Poppy seed
The tiny, oil-rich seeds of the opium poppy, blue-grey and nutty. Hungarian cooking mostly uses them freshly ground and in large quantities.
Whole poppy seeds are sold in most supermarkets, but ground poppy seed in larger packs is easier to find in Eastern European shops, health food stores or online. Since the oil turns rancid quickly once ground, it is best to grind the seeds yourself just before use in a poppy seed mill or a clean coffee or spice grinder.
Substitute:
There is no real substitute; ground walnuts make a good filling, but a different dish without the characteristic earthy, slightly bitter poppy flavour.