In the Levant, zaatar is listed by Wikipedia as a possible croissant filling.
Zaatar is a Levantine herb mixture traditionally built around dried oregano-like herbs, toasted sesame seeds, salt, and often tangy sumac. It is especially associated with manakish, a flatbread topped with zaatar and olive oil, but its savory, aromatic flavor also works in filled pastries.
Croissants in the Levant may be sold plain or filled with chocolate, cheese, almonds, or zaatar. That variety can surprise readers who associate croissants only with butter or sweet fillings. The zaatar filling is not a claim that every croissant contains it; it is a regional variation.
The name also causes confusion because “zaatar” can mean both the herb plant and the prepared spice blend. Spellings include za'atar, zaatar, za'tar, and zatar, but they refer to the same culinary tradition in this context.