Iranians love to feed people. No matter if you have already eaten or not, an Iranian will always try to make you eat something and will use the expression:
نوش جان / nooshe jan / bon appetit
Nooshe jan literally means may your soul be nourished. It can be said at the beginning or end of a meal. At the beginning, it means bon appetit, and at the end, it's the cook's reply after you thank them. "The food was so delicious, thank you!" "Nooshe jan!"
Imagine you are eating pistachios. You taarof, offer, some to your friend, but they don't want any. They would say, "No, thank you, but nooshe jan." Or they might simply shake their head and say nooshe jan, and you know that they don't want any.