In Hilchot Shabbat, the vessel that was heated on the fire is called “Keli Rishon”-the first vessel. If the hot contents were poured into a second vessel, that vessel is called a “Keli Sheni.” Even if the contents of the Keli Sheni are still be very hot, the Halacha states a general principle that “Keli Sheni Enno Mevashel”-putting raw food in the second vessel does not constitute cooking on Shabbat. For example, if one poured hot water from the urn into a teacup, the teacup is a Keli Sheni, and therefore it is permitted to put a raw lemon inside the hot water. This is the conclusion of Hacham Ovadia and Hacham Benison.
The question is why doesn’t the Keli Sheni cook? Aren’t its contents also very hot, like the Keli Rishon? Tosafot (Shabbat 40b) explain that although the contents may be hot, the walls of the vessel are cold and this allows it to cool rapidly, dampening its ability to cook. Based on this, the Maharshal (R. Shlomo Luria, 1510-1573), as understood by the Shach, writes a phenomenal Chidush: If there would be a Keli Sheni that was not in contact with the cool walls of the vessel, it would cook a raw food placed on it. That is, if one took a potato or a piece of meat from a Keli Rishon and placed it on a plate, which is the Keli Sheni, the meat is not being cooled by the walls; it is standing alone as a “Davar Gush”-a solid mass. Consequently, the piece of hot food is treated as a Keli Rishon, and it would be prohibited to apply raw spices or other raw foods to the hot food on the plate.
The Rema (in Shulhan Aruch and in Torat Hatat) looks at this in a different way. He maintains that if the piece of hot food is placed in a Keli Sheni, then it also has the status of Keli Sheni and cannot cook another raw food. Accordingly, one may pour cold gravy over the hot food, since it is only a Keli Sheni.
The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933, in Siman 318:45) and the Kaf HaHaim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) rule in accordance with the Maharshal that a “Davar HaGush” does cook in a Keli Sheni. On the other hand, Hacham Bension (Vol. 2) writes that the custom of the Sepharadim is to be lenient and not distinguish between a liquid and a solid mass in a Keli Sheni. This is also the opinion of Hacham Ovadia in Hazon Ovadia (Vol. 4). Hacham David, in his new Halacha Berura, also brings this position, although he adds, “Hamachmir Tavo Alav Beracha”- it is praiseworthy to be strict.
One may pour cold gravy or raw spices on a hot solid food in a Keli Sheni.