Let's start with some background. There are a couple of potential issues when reheating food on Shabbos: Bishul which may be a Torah prohibition and leaving cooked food on the fire and the appearance of cooking which are Rabbinic in origin.
1) If the food was fully cooked and is currently dry without any significant quantity of gravy etc., there is no risk of Bishul on Shabbos. Therefore, meat without gravy, potato kugel and similar foods do not entail a Biblical prohibition if they were completely cooked before Shabbos.
2) However, there are two potential Rabbinic issues. Just as cholent and other foods may not be left on the stove from before Shabbos, so too items may not be placed on a fire on Shabbos. This is primarily due to a concern that one may stoke the fire or otherwise adjust the temperature to speed up the heating.
This problem is also easily solved. If the fire is covered and the temperature cannot easily be adjusted, the concern is alleviated. So, the blech we place the cholent pot on would be an acceptable location for the kugel, as both the fire and knobs are covered and we have a precaution against mistakes. The same would apply to an electric hotplate that does not have multiple temperature settings.
3) The second Rabbinic issue is the appearance of cooking. Since one could conceivably cook raw food on a blech, someone who sees you placing a pan on the blech might not realize it is cooked food and imagine that you are cooking raw food on Shabbos. This mistaken assumption could lead to tragic consequences, as the misguided observer may jump to the unsubstantiated conclusion that one is permitted to cook raw food on a blech on Shabbos, and do so in their own house.
To prevent this problem and remove the Rabbinic injunction, the reheating must be done in a manner that does not resemble the cooking of raw food. The electric hotplate is a matter of dispute among the Poskim, as many of them reach a sufficient level of heat to theoretically cook raw food, but it is certainly not a common practice. Some consider this sufficiently "unusual" while others require a warmer that does not reach a temperature capable of cooking. In any event, for a blech this is not a solution.
A universally accepted solution for a blech or hotplate is to place the food to be warmed on a part of the blech that is not hot enough to cook. Alternatively, the food may be placed on top of a different pot filled with food. For example, the pan of kugel may be placed on top of the pot of cholent (suggestion, turn the lid upside down to prevent the kugel from sliding off). If this is impractical, an empty pan may be placed upside down upon the blech/hotplate, and the food to be reheated on top of this pan.
All these methods require more than the usual time span to warm the food, and it takes a little practice to determine how early Shabbos morning you need to put up the food.