shalom rav a few questions on bishul:
1) can one warm up bread on shabbat or does one have to be doubtful of toasting?
2) why is it necessary to place a hard boiled egg in a kli sheni ot warm up if it was lready cooked? why not keli rishon?
3) can one use any untensils for hefsek kedeyrah? even inverted tinfoil-tray? and specific specifications?
4) is a crock-pot considered a ‘blech’? can one leave raw chulent in a crock before shabbat to continue to cook? can one leave anything raw on a blech to cook thruought shabbat?
1) If the bread is placed somewhere so that it can’t turn into toast, for instance on a heater, it is permitted. See Iggros Moshe Orach Chaim 4:74:34; Otzros Shabbos p. 101, citing from Shevet Ha-Levi; Orchos Shabbos 1:p. 100. See also Yevakesh Torah, Shabbos, p. 172, who writes at length concerning whether one must be concerned for the hardening of the challah (and its becoming toast-like), and concludes that one may be lenient in this matter.
2) If the egg is fully cooked, it may be placed in a keli rishon, provided it is off the fire.
3) Some permit the reheating of cold and dry food that is fully cooked on top of an inverted pot. Although other authorities are stringent, requiring specifically a full pot of food, many are lenient to follow the former opinion. Concerning using an upside down tray/pot in general, see Shemiras Shabbos Kehilchasah note 112; Shevet Halevi 1:91; and see also http://www.dinonline.org/2011/02/03/shabbos-hatmana-returning-food-to-heat-on-shabbos/, note 12-13. Provided the tinfoil tray is strong enough to hold the pot on top of it, this is fine according to the lenient opinion.
4) A crock pot is not considered a blech, and uncooked food cannot be left in it from before Shabbos.