And they shall eat the meat on that night, roast with fire and matzot; with bitter herbs they shall eat it.
Do not eat of it raw nor boiled in water; but roast it with fire, its head with its legs and with its inner parts.
The seventh perek of Massekhet Pesahim deals with how the korban Pesah is eaten. Perhaps the most basic rule about the preparation of the sacrifice is that the animal must be roasted whole. One of the concerns raised in the Gemara is the issue of removing the blood from the meat before it is eaten.
The prohibition of eating blood appears a number of times in the Torah, along with a severe punishment – karet (excision). Even though the only blood that is forbidden on a Torah level is the blood that comes from the animal at the moment it is slaughtered, nevertheless, due to the severity of the prohibition we try to remove as much of the blood as possible before cooking it and eating it, which is why kosher meat is generally salted. According to the Sages, salt has the power to absorb the blood and to actually draw the blood out of the meat before it is washed off.
According to the letter of the law, as long as the blood remains in its place in the meat, it is not forbidden. Therefore, a person would be allowed to eat raw meat (referred to by the Gemara as umtza) even if it was not salted. The moment such meat is cooked, however, the heat would draw the blood into the water, which would be forbidden.
In theory, there are ways to “freeze” the blood in its place in the meat, for example by placing it in a strong vinegar solution, which would then allow the meat to be cooked, since the blood would never leave the meat. Already during the times of the Ge’onim this method was forbidden.
Another method that is recommended for removing blood is roasting. The heat of the fire acts as an agent to draw the blood from the meat, so there is no need to salt the meat at all, although tradition has it that a small amount of salt is sprinkled on.