Ha’Rav Ben Tzion Aba Shaul (a”h), in his sefer, “Ohr Le’Tzion”, states that according to the “Igle Tal,” (A great Rabbi of his time), there is room to be lenient concerning this question and one may indeed pour hot water from an urn or kettle into a pot of food that is on the blech or hot plate. In addition, Rabbi Moshe Ha’Levi (a”h) in his sefer “Menuchat Ahavah,” wrote that it was permissible.
However, Ha’Rav Ovadyah Yosef (a”h) wrote that according to Rabeinu Yonah (a”h), it is forbidden to pour this water into a pot even though it is already cooked and is coming from an urn. He says that the water will cool down during the transfer process, and therefore it would be forbidden because the water will be cooked again.
He is therefore stringent in this case. However, be’diavad, the food is not forbidden to eat if one does pour hot water onto it while it is on a blech or hot plate. One has what to rely based on the lenient responses provided by Ha’Rav Ben Tzion and Rabbi Moshe Ha’Levi.