And the 56th mitzvah is that we are commanded to eat the Pesach lamb on the night of the 15th of Nissan, keeping all the conditions which are stated, i.e., that it be roasted, that it be eaten in one house, and that it be eaten with Matzah and bitter vegetables.
The source of this commandment is G‑d's statement (exalted be He), "Eat the meat on this night, roasted over fire. With Matzah and bitter vegetables you shall eat it."
Perhaps a questioner will challenge me, "Why do you count eating the Pesach [offering], Matzah, and bitter vegetables as a single commandment rather than as three commandments? Eating Matzah is a mitzvah, eating the bitter vegetables is a mitzvah, and eating the Pesach offering is a mitzvah!"
I will answer him that, "eating Matzah is a separate commandment," is correct, as we shall explain. It is also correct that eating the meat of the Pesach offering is a commandment, as we mentioned. But the bitter vegetables are secondary to eating the Pesach offering, and are not counted separately.
The proof of this is that it is a mitzvah to eat the meat of the Pesach offering whether or not there are bitter vegetables. But the bitter vegetables are not eaten unless there is meat from the Pesach offering, as it is written in G‑d's statement (exalted be He), "Eat it with Matzah and bitter vegetables." If one ate the bitter vegetables without the meat from the Pesach offering, he certainly accomplished nothing; we do not say that he fulfilled [at least] one commandment, i.e., that of eating the bitter vegetables.
In the words of the Mechilta, "The verse says, '...roasted over fire, with Matzah and bitter vegetables.' This verse teaches that the commandment of the Pesach offering is roasted meat with Matzah and bitter vegetables," i.e., that the [single] commandment includes all of them.
It [also] says there, "From which words do we derive that those who do not have Matzah or bitter vegetables fulfill their obligation [just] by eating the Pesach offering? From the [apparently extra] expression, 'you shall eat it,'" — i.e., the meat by itself.
"One might think that just as those who lack Matzah and bitter vegetables fulfill their obligation [just] by eating the Pesach offering, so too those who lack the Pesach offering fulfill their obligation [just] by eating Matzah and bitter vegetables. One might make the following comparison: since [eating] the Pesach offering is a Positive commandment and [eating] the Matzah and bitter vegetables is a [separate] Positive commandment, once you learn that if they lack Matzah and bitter vegetables, they fulfill their obligation [just] by eating the Pesach offering, so too, if they lack the Pesach offering, they fulfill their obligation by eating [just] Matzah and bitter vegetables.
[To teach that this comparison is wrong,] "The Torah therefore says, 'you shall eat it.' "
It also says there, "The expression, '[With Matzah and bitter vegetables] you shall eat it,' teaches that the Pesach offering is eaten when one is full, but the Matzah and bitter vegetable are not eaten when one is full." This is because the [primary] commandment is to eat the meat [of the offering], as it is written, "Eat the meat on this night."
[In conclusion,] the bitter vegetables are secondary and requisite to the Pesach offering, as clear from the above quotes to anyone who understands them.
The clear proof of this is the basic principle given in the Talmud, "[Eating] bitter vegetables today is [only] a Rabbinic obligation." This is because there is no Torah obligation to eat it by itself; it is eaten [only] if there is meat from the Pesach offering. This is a clear and obvious proof that it is secondary to the commandment, and that eating it is not a separate commandment.
The details of this mitzvah too are explained in tractate Pesachim.
