The Shulhan Aruch (253:4) protests against the practice of some people to pour hot water heated before Shabbat on hot foods on Shabbat to prevent them from drying up. His ruling is based on Rabbenu Yonah. Rabbenu Yonah gave two reasons for this prohibition. First, the water or the food may have cooled off, in which case the hot one will be cooking the cool one. Rabbenu Yonah also argues that even if one of the two have not cooled off previously, the act of pouring water, by definition, cools the water as it moves through the air, and it becomes a Keli Sheni, which is subject to being cooked by the hot food.
The Poskim discuss which of these two reasons was adopted by Maran. Hacham Bension understands that the first reason justifies the prohibition. Therefore, if both the water and food are on the Blech, and one is certain they did not cool off, he may pour the hot water from the kettle on the Hamin to prevent it from drying up. On the other hand, Hacham Ovadia, in Hazon Ovadia and Yehave Da’at, writes that Maran also takes the second reason into account. Therefore, in all cases the hot water cools off and becomes a Keli Sheni and is subject to being cooked by the food.
It is prohibited to pour hot water on a hot food on Shabbat.