Za’atar is a Middle Eastern spice blend with a history of use dating back to medieval times. Nowadays, a typical blend for za'atar consists of cumin, dried thyme, oregano, marjoram, sumac, toasted sesame seeds and salt.
Spice mixes are always best used within 1 - 2 months for maxiumum flavour and freshness so avoid making very large batches that cannot be used in time.