i have a warming drawer that is a separate apparatus from the oven, which has a knob options of low, medium and high, which I cover on shabbos; and when I put in food that I can leave slightly uncovered due to hatmana; can this warming drawer be used to reheat ‘cold-solid-already cooked food’ on shabbos day and do the settings (low,medium,high) matter in this regard?
If the food is solid and fully cooked, and if the apparatus is such that it is not possible to use it for cooking (in an effective manner), then some (Iggros Moshe, vol. 4, no. 74, sec. 34) permit its use for heating food on Shabbos, provided the food is fully cooked and solid. However, the knob must be covered over.
Sources: See Beis Yosef and Shulchan Aruch, Orach Chaim 253:2-3, and Biur Halachah (s.v. Yizaher). According to Iggros Moshe, there is no prohibition in placing food, even cold, on an apparatus that cannot be used for cooking, yet he prohibits doing so when there are two settings. However, many poskim do not follow this leniency (see Shemiras Shabbos Kehilchasah 1:25), and many are stringent with regard to the common hot-plate not to place a cold food lechatchilah on Shabbos. Shabbos Kehalachah (10:14-15) writes that one should preferably place an inverted pot on the hot plate, and place the food on the inverted pot.