Can you put a flat metal tray upside down on a blech and then warm up cold dry food on Shabbat by placing it directly on the tray?
Yes you ay, however the food has to be a dry food, and not one with sauce etc. as the sauce can get recooked when it is reheated.
Shmiras Shabos Khilchoso 1 ftnt. 126 (new edition)