Put the sesame seeds in a small dry skillet over medium heat. Toast the seeds, tossing the pan occasionally, until they become fragrant and start to lightly brown. Immediately transfer them to a bowl to cool; if you leave them to cool in the pan, they could burn. Stir in the sumac, thyme, cumin, and salt. Transfer to a small jar with a lid and store in a cool, dark place for up to 6 months.