The [Israelites] are to eat the lamb, together with unleavened bread and bitter herbs.
— Numbers 9:11

Rabbi Hillel the Elder (who lived one generation before the 2nd Temple was destroyed by Roman Imperial General Titus) used to wrap the lamb and bitter herbs and unleavened bread together. Generations of rabbis then argued about dipping or not dipping this into charoset as well as including or leaving out the lamb.

Remembering the affliction, the bitterness and the hardship, make the Hillel sandwich.

haggadah Section: Koreich
Source: The Minimalist Haggadah by Jon Kessler