Q. Is Sous-Vide (under vacuum, see above question) cooking considered cooking for bishul akum or mevushal on Shabbos purposes?
May one leave it overnight on Shabbos or is it prohibited hatmana?

A. Horav Shlomo Miller’s Shlit’a opinion is that although unusual cooking such as cooking by hot smoke or in the sun’s rays was not included in the bishul-Akum decree, sous-vide is considered cooking for becoming prohibited as bishul Akum.
Since the cooking temperature is above 45º C. It is also considered mevushal for Shabbos purposes.
As far as hatmana is concerned Chazon Ish (37: 32) maintains that there is no hatmana prohibition when placing food in a utensil inside the food contained in another vessel, such as a plastic bag or aluminum foil containing kugel or kishke inside the pot of tcholent before Shabbos. He adds that the two containers are considered as one. Some Poskim maintain that this is only permitted when the inner container is not completely covered by the food in the pot. (Ohr Hameir p. 76 quoting Taz s. 258)
Horav Shlomo Miller’s Shlit”a opinion is to be lenient since the inner plastic bag is not covered completely.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a.