Adding Water to Dafina
The Shulhan Aruch (Orah Haim 253:4), based on Rabbenu Yonah, says that the practice of adding hot or boiling water from an insulated urn to to a cooked dish on Shabbat to prevent it from drying up should be protested. (Adding cold water, according to the Ashkenazic community and adding warm water, according to the Sephardic community is expressly forbidden). Rabbenu Yonah says that one reason for this stringency is because sometimes this water may have cooled down, and one may violate the prohibition of cooking on Shabbat. Another reason is that when one pours hot water from the urn, which is a primary vessel, the water leaving the walls of the urn is considered to have come from a secondary vessel and by contacting the hot food, the water becomes cooked.
One practical application of this is adding hot water from a hot water urn or a teapot into Dafina (also known as Hamin or Skhina, and is akin to Cholent) to improve its consistency. Rabbi Ovadia Yosef (Yehave Da’at, vol. II, § 22) rules like Rabbenu Yonah and says that one should not add hot water to one’s Dafina on Shabbat. Nevertheless, Rabbi Messod HaKohen (Pirhe Kehuna, § 33), Rabbi Yosef Messas (Mayim Haim, vol. I, § 152), Rabbi Shalom Messas (Tevuot Shamesh, Orah Haim, § 26) and Rabbi Moshe Malka (Mikve HaMayim, vol. III, § 42) all write that the Moroccan custom is to permit adding hot water to Dafina on Shabbat. If one examines the commentaries to Rabbenu Yonah’s opinion quoted in the Shulhan Aruch, namely the Magen Avraham, the Taz, the Mishna Berura, the Aruch HaShulhan and the Kaf HaHaim (all ibid.), the main concern to forbid adding hot water to cooked food is that at times one may use cold water. Rabbenu Yonah was referring to a situation in which one takes a kettle off a heat source and insulates it, and at times may come to use a kettle with cold water. Nowadays, however, one generally takes hot water from a heat source, such as a hot water urn or a an oven and immediately pours it into the Dafina. As such there is no concern of using water that is cold.
Regarding Rabbenu Yonah’s other reason, the Hazon Ish (§37:13) explains it as follows: When hot water is poured from a primary vessel into a secondary vessel, it Halachically loses its ability to cook something else (even though in actuality, the water is still very hot). This rule only applies to the water’s ability to cook, but when it comes to the water’s ability of being cooked, even though it is in a secondary vessel, it is considered Halachically as hot as when it was in the primary vessel. Therefore, if one were to pour water from a secondary vessel into Dafina, it would not be as though the water was being cooked, and as such it would be permitted. Rabbi Ben Zion Abba Shaul (Or LeZion, vol. II, ch. 17, § 8) concurs and says there is a Halachic basis to adding hot water to a cooked food which is on a heat source on Shabbat.
Summary: The Moroccan custom is to permit adding boiling water to Dafina on Shabbat to prevent it from burning or to improve its consistency.