| Of course, this isn't what Rabbi Hillel invented. The Hagaddah, read on Passover, specifically explains (my translation) that "Rabbi Hillel would bind Pessach, matzah and bitter herbs". Pessach is the portion of the passover sacrifice that is eaten.
What Hillel invented was the roast lamb and horseradish sandwich more than anything else. Nowadays (and for the past 2000 years) nobody has access to the passover sacrifice. And the combination of matzah (unleavened bread), horseradish and haroset (a sweet blended mixture of dried and fresh fruits, made in various ways depending on where your folks came from) is really good, so we eat that instead, and claim Rabbi Hillel invented it.
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