Is it permissible to heat a slice of pizza on a warming tray or blech on Shabbos?
ב"ה Shalom If the pizza is a fully baked pizza but became cold in the fridge or freezer, and you want to warm it up, you may warm it on the warming tray in the same manner you would warm a piece of cooked chicken, because when solid food that was fully cooked or baked is warmed, it is not considered that they are being cooked again. If you put it on the blech it must be place on something else but not directly on the blech. Many hold that the same applies when placing cold food on the warming tray, that the food should be placed on top of some other dish so as it not should look like on is cooking on Shabbat. (See Shmirat Shabbat Kehilchata) The prebaked pizza may be placed on the warming tray even if the crust becomes a bit crisp on Shabbat, the same way many people place a Challa from the freezer on the warming tray on Shabbat . (יחווה דעת ח"ג סי' כב,). All the Best