The Halacha discusses whether one may place a raw food on Shabbat in a "Keli Sheni"-the second vessel containing hot liquid after it was poured from the first vessel which was on the fire. Seemingly, the Halachic principle that a "Keli Sheni does not cook" would dictate that there is no problem. Nevertheless, the Tosafot (Shabbat 39) suggest one approach, based on several proofs, that even though the Keli Sheni cannot actually cook a raw food, there is still a problem of "Mehezeh K’mevashel"-it appears that one is cooking. After all, the water in the Keli Sheni is still piping hot. However, in their second approach, the Tosafot maintain that there is no problem whatsoever of placing a raw food in a Keli Sheni-not even "Mehezeh K’mevashel." Accordingly, they re-learn the various sources which were previously understood to be proofs to the contrary.
The Bet Yosef (318) brings both answers of Tosafot and does not explicitly rule in accordance with either one. Thus, there is uncertainty as to what Maran’s opinion is. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) understands Maran to be strict and prohibit placing raw food in a Keli Sheni, because of "Mehezeh K’mevashel." This is also the understanding of the Mateh Yehuda Ayash and Hacham Bension.
On the other hand, Hacham Ovadia, in Yabia Omer and Hazon Ovadia (Shabbat vol. 4), is lenient and allows heating raw food in a Keli Sheni, without concern that it appears to be Bishul-cooking. Hacham David in the new Halacha Berura explains that his father is relying on a "Safek Sefekah"-a double doubt. First, perhaps the Halacha is in accordance with the opinion that one may even pour hot water on raw food directly from a Keli Rishon and certainly from a Keli Sheni. Second, even if the Halacha is not in accordance with that opinion, perhaps the Halacha does agree with the second lenient opinion in Tosafot that is not concerned with "Mehezeh K’mevashel" with a Keli Sheni. The Menuhat Ahaba (2:10) concurs with this opinion, and argues that the second approach of Tosafot, which in this case is lenient, is always more authoritative, since there must have been a fault with the first approach. This is the accepted Halacha for Sepharadim
Based on this, it is permitted to put a raw lemon or Nana mint leaves in a cup of hot water poured from the urn. Similarly, one may pour milk into a Keli Sheni.
Ashkenazim are stricter in this matter and have limitations in these applications.
It is permitted to place raw food in a Keli Sheni on Shabbat.