In recent years, the Slow Cooker has become very common in Israel, especially for "cholent," stew which is left to simmer overnight, to be eaten Shabbat morning. The device consists of two vessels. The inner one is a heavy pot (usually earthenware), which is directly brought to the table. This is kept hot overnight in an outer electric vessel which covers the sides and bottom of the pot (but not the top).
There are those who forbid using such a device on Shabbat because of the prohibition of hatmanah – covering a dish – in a way that "increases the heat." On the other hand, many rabbis permit using this device because the pot is not completely covered, since the top remains in the open. The following are some instructions related to using this type of equipment:
a. The cholent should be cooked before Shabbat up to the point that it is edible (at least one-third of full cooking, defined as "food fit for Ben Derusai"), or as an alternative some raw meat should be placed in the pot right before Shabbat (if it is completely raw when Shabbat starts, the fact that the meat is cooked before it is eaten the next morning is not a Shabbat desecration).
b. Before Shabbat starts, the control switch should be moved to "low" or "high" but not to "automatic", which usually involves a thermostat. The reason is that removing the inner pot causes the walls of the external vessel to cool down, and this can cause the thermostat to immediately activate the heating element.
c. An alternative is to set the switch to "automatic" and to use an external timer to disconnect the cooker a short time before the inner pot will be removed (this provides the additional benefit of saving some electricity).
d. The top of the pot should not be covered with a towel or a cloth, to avoid violating the prohibition of hatmanah.
e. Those who want to be stringent (defining the situation as hatmanah even if the top of the pot is not covered) can lift the inner pot slightly (about one cm) by putting small objects underneath it. In this way, the side of the pot extends slightly over the edges of the outer heating vessel.
f. Since this equipment is a pot and not a Shabbat hotplate, the inner pot should not be returned to the slow cooker on Shabbat (because this will "give the appearance of cooking"). If not all of the food is needed at first, a large spoon can be used to remove some of it from the pot, even according to the Ashkenazi custom which usually forbids taking food out of a hot pot because of the danger of mixing it (according to the opinion of the Chazon Ish). In any case, it is recommended to set a Shabbat timer to turn the electricity off while the food is being removed from the pot.