The Mishna in Masechet Pesachim tells that some communities had the custom not to eat roasted meat on the night of the Seder. People who see others eating roasted meat might mistakenly assume that they are partaking of the meat of the Korban Pesah, which had to be roasted and could not be prepared any other way. This might leave people with the impression that it is permissible to eat sacrificial meat outside the Bet Ha’mikdash. Therefore, some communities accepted this custom to refrain from roasted meat on the night of the Seder.
The Ashkenazim, as noted by several Halachic authorities (Mishna Berura, Aruch Ha’shulhan), have adopted this custom, and the work Derech Eretz documents that this was the custom in Halab (Aleppo, Syria), as well. However, in a footnote, the Derech Eretz cites a book called Bet Ha’behira stating that the Jews of Halab permitted eating roasted meat on the night of the Seder. Regardless, the accepted practice among our community today is to refrain from eating roasted meat at the Seder. Meat that was boiled in water, even after it was roasted, or that was roasted in a pot ("Seli Kedar"), is permissible. Therefore, the Zeroa, which is placed on the Seder plate, and which many people have the custom to eat before or during the meal at the Seder, should be boiled in water after it is roasted, so it may be eaten. This is the ruling of Hacham Bension Abba Shaul (Israel, 1923-1998) and of Hacham Ovadia Yosef.
This Halacha applies to any food prepared by slaughtering, and thus it applies to chicken and red meat alike. It should be noted that liver only becomes permissible for consumption through roasting, and so one who wishes to eat liver at the Seder should boil it after it is roasted. Roasted eggs, fish and vegetables are permissible.
This custom applies only on the night of the Seder, when the Korban Pesah would be offered; during the day of Pesah it is entirely permissible to eat roasted meat.
The Rama (Rav Moshe Isserles of Cracow, 1525-1572) writes (in Siman 476) that it is customary to eat an egg at the Seder. Various reasons have been given for this custom, one of which is that the first night of Pesah always falls on the same night of the week as Tisha B’Ab of that year. This is alluded to in the verse, "Al Masot U’mrorim Yocheluhu" – the night of Masa is also the night of "Merorim," the bitterness of the Temple’s destruction. As eggs are traditionally eaten by mourners, we eat eggs at the Seder as a reminder that we are in exile.
The Halachic authorities caution that although one should eat heartily at the meal of the Seder, he must ensure not to eat so much that he is too full to eat the Afikoman. One does not fulfill the Misva by eating "Achila Gasa" – when he has no appetite at all – and therefore one must ensure not to eat too much during the meal. Additionally, one should not drink wine excessively during the meal, as he may then become inebriated and thus be unable to properly fulfill the Misvot of the Seder.
Summary: It is customary not to eat roasted chicken or meat at the Seder, unless it had been boiled in water after it was roasted. Therefore, the Zeroa, which many people eat at the Seder, must be boiled after it is roasted. Other roasted foods may be eaten at the Seder. It is customary to eat eggs at the Seder. One must ensure not to eat so much during the meal of the Seder that he has no appetite for the Afikoman.
