Maran (Shulhan Aruch 253:5) discusses reheating a food called 'panadish' on Shabbat. He rules that it is permissible to return such a dish to the blech even though it has cooled off because it is a solid, fully cooked food. However, the Mishna Berura (Rav Yisrael Meir Kagan, 1839-1933) points out that if this dish contains a significant amount of congealed fat, it would be forbidden to reheat it, because the solid fat will melt into a liquid. This constitutes the prohibition of Nolad, creating a new form of a substance.
Maran, however, in siman 318:16 clearly states that it is permitted to reheat a dish called 'empanada' even though it also contains congealed fat which will melt. Evidently, he holds that there is no problem of Nolad. The prohibition of Nolad is only present if one actively intervened to change the form of the food. For example, Crushing ice with one’s hands and transforming it to water would be a problem of Nolad. In our case of reheating congealed fat, the person only put the food on the fire and the melting happened by itself. Ashkenazim, on the other hand, are much more machmir on this issue than the Sepharadim.
The other potential issue with reheating a food with congealed fat concerns the prohibition of reheating cold liquids. One could claim that the melted fat constitutes a liquid which is forbidden to reheat. Hacham Ovadia says that this is not a problem (Hazon Ovadia Vol. 4). He explains that the melted fat is not considered a liquid, because we look at the state of the food when it was originally placed on the fire. At that time, it was still solid, and therefore, it is permitted to reheat. He cites a precedent from the Shlah who says that it is even permitted to put a stick of butter on the blech even though it will soon entirely melt into a liquid.
Summary: It is permitted (according to the Sepharadim) to put a dry solid food containing congealed fat back on the blech on Shabbat.
