i want to slow cook/bake something for more than a day (in a gas grill with the temperature of 180F)
i was told i should not put it to cook on Friday for it to be ready on Sunday
is this true?
what i the reason? (sources please)
The prohibition against work does not extend to your vessels working, and therefore you vessels are allowed to do work on Shabbat (See Halacha 15 where the Rabbis prohibited noisy vessels from doing work, because of Zilzul Shabbat - Disrespect? of Shabbat - because people will hear the noise and think you did Melacha on Shabbat).
The second Halacha there talks about dying wool on Shabbat (a process which involves soaking the wool in boiling dye for long periods of time). While it would theoretically be permissible to do this over Shabbat, if it was started before Shabbat, there are other factors that prohibit this on Shabbat (these concerns are about cooking, and might therefore apply to your questions):
We are worried you might come to stir the coals the pot is sitting on.
Even if there are no coals, we are worried you might take off the lid and stir the dye/wool, and stirring is considered cooking if the food is still in the original pot it was cooked in (even if it is no longer on the fire).
Even if the wool was already fully dyed before Shabbat, stirring it would still be a problem of dyeing on Shabbat. There is a work around for this. If the pot is sealed with clay, we are not worried that you will break the seal, remove the lid, and stir the contents.
Based on this, one could posit that it would be permissible to put food on the stove before Shabbat, intending to have it sit on the fire the whole Shabbat, and eat it after Shabbat, if the following conditions are met:
The food is on a blech.
The food was already cooked before Shabbat. (this is based on my understanding of 252:2. Based on 254:1, it is possible that as long as the pot is sealed, the food does not be to be fully cooked)
The pot is sealed.
See also 254:11, which talks about putting bread in the oven before Shabbat intending to let it bake over Shabbat and eat it Motzei Shabbat. Because one may change his mind and decide to eat it on Shabbat, and in order to do so will try to speed up the cooking, one may only do this if he uses a blech.
As always, CYLOR
H"T to the great DOUBLEAA
The Shulchan Aruch bring the minhag of the Ashkenazim not to do it, (the Mishnah Berurah says bidieved you do not need to wait kdai sheyaasu).
The Shulchan aruch says this regarding yom Kippur (h"t to sabbahillel) but all the guesses of reasons (below) seem to include shabbos too
Aruch hashulchan 609
Shulchan aruch harav