Question: If my cholent dries out on Shabbos, may I add hot water to the pot?
Answer: The Shulchan Aruch (OC 318:4) writes that one can pour hot water into a hot dish that has already been cooked on Shabbos. Yet, elsewhere, the Shulchan Aruch (OC 253:5) writes that it is forbidden to pour hot water from a kettle into a pot of food. The Beis Yosef (OC 253:15) explains that we are worried that the dish has cooled down and it is now being ‘cooked again’ by the hot water (or vice versa). Thus, providing both pots are hot, it should be permitted to pour from one to the other.
Nonetheless, R’ Benzion Abba Shaul (Ohr Letzion 2:17:8) writes that there are other reasons why the Shulchan Aruch writes this halacha and one shouldn’t add hot water to the pot regardless as to whether it’s on the stove or not. Thus, R’ Ovadia Yosef (Yechave Daas 4:22) writes that sefardim should ideally not add hot water to hot dishes.
R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 1:16) writes that one may transfer the contents of one pot to another while they’re both on a blech (See Mishna Berura 318:84). Quoting R’ Shlomo Zalman Auerbach (1:n44), he writes that ideally one should pour the water directly from the urn, though if necessary one can use a cup (kli sheni) to pour the water providing the water is still hot (See Minchas Yitzchak 6:20; 10:18; Rivevos Ephraim 1:246).
As one cannot stir the pot while it’s on the flame (maygis), one should pour the water in slowly (See Rambam, Shabbos 9:4) or remove the pot from the flame before adding water.In conclusion, while many sefardim avoid doing so on Shabbos, ashkenazim may add hot water to a hot cholent pot on Shabbos providing that the food is fully cooked and is on a blech.