ON THE SECULAR SIDE:
Rav Ammi, who says Charoset is an antidote for "kappa" which is either a worm or a poison common to vegetables, or a type of indigestion caused by vegetables.
ON THE RELIGIOUS SIDE
R' Eliever ben Zadok
R. Levi, who says we eat it to remind ourselves of the "apple" tree alluded to in Songs 8:5. The idea is that women defied the Egyptian decree by continuing to give birth "beneath the apple trees."
R. Jonanan , who says we eat it to remember the mud, used to make bricks
So, R. Levi sees charoset and remembers the heroism of Jewish women; R. Jochanan looks as the same stuff and recalls how our ancestors suffered.
Can you imagine the same thing happening with ketchup?
R. Levi said: In memory of the blood of circumcision; R. Johanan said: In memory of the blood of the plagues.
Or with duck sauce?
R. Levi said: In memory of how thin we were [=דק] ; R. Johanan said: In memory of the horses that perished in the Red Sea [=סוס]
Expanded version of this post with sources from 2012 found here
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