One of the most popular songs at the Passover Seder is Dayenu (it would have been enough for us). Many of the concepts in Dayenu come from Parshat Beshalach:
If God had only given us their treasures and had not divided the sea- Dayenu
If God had only divided the sea and had not led us across on dry land- Dayenu
If God had only led us across on dry land and had not drowned the Egyptians- Dayenu
If God had only drowned the Egyptians and had not taken care of us in the desert for forty years- Dayenu
If God had only taken care of us in the desert for forty years and had not fed us the manna- Dayenu
If God had only fed us the manna and had not given u s the Shabbat-Dayenu
There is a custom among Persian Jews to hit each other with green onions when they sing “Dayenu” after the words: “If God had only taken care of us in the desert for forty years and had not fed us the manna.”
The Haggada called A Different Night suggests that this custom may be based on the fact that B’nai Yisrael complained about the manna and said (Bamidbar 11:5-6) “We remember the fish that we used to eat in Egypt freely, the cucumbers, the watermelons, the leeks, the onions and the garlic. Now our bodies are withered. There is nothing at all, nothing but this manna to look at.”
By hitting each other with onions, the participants at the Seder are showing how ashamed they are that B’nai Yisrael were so ungrateful and unappreciative when they were in the wilderness. Instead of being happy that they had something to eat, they were yearning for the food in Egypt, forgetting about the fact that when they were slaves they were hardly given any time to eat.
Dayenu is the part of the Seder when we should think about how grateful we should be for what we do have and not dwell on what we don’t.