Blech

A blech (from the Yiddish word (blekh) meaning "tin" or "sheet metal") is a metal sheet used by many observant Jews to cover stovetop burners (and for some, the cooker's knobs and dials) on Shabbat (the Jewish Sabbath), as part of the precautions taken to avoid violating the halachic prohibition against cooking on the Sabbath.

Rabbi Fishel Jacobs' The Blech Book—The Complete & Illustrated Guide To Shabbos Hotplates gives the following guidelines:-

During Shabbos, the pots are removed according to need. After removal, it is permissible to return the pot onto the blech, following these guidelines:

The permissibility of blech (and unblech, below) and the acceptable manner of their use is questioned by several modern kashrut organizations; however, the use of a blech to reheat food on the Sabbath remains very popular among observant Jews.

An unblech, or K'Deira Blech (lit. "pot blech", commonly referred to as "water blech"), is also used to heat up pre-cooked food on the Sabbath, but utilizes different halakhic mechanisms from a standard blech. An unblech consists of a shallow metal pan filled with hot water and covered by another metal pan, and thus is akin to a pot of warm food for halakhic purposes. As such, it may be more flexible than a standard blech for halachic purposes. However, the temperature of an unblech is limited by the boiling point of water and is not as hot as a typical blech.

It is always important to have working smoke detectors in a house and the use of a flammable blech cover should be discouraged.

In 2015, a house fire caused by a faulty Shabbos hot plate killed seven children in Brooklyn, New York City, New York. The 2015 fire was preceded by at least four other Shabbat fires in Brooklyn in the past 15 years caused by appliances for heating food being left on or candles burning during the Jewish Sabbath in order to comply with Orthodox interpretation of Jewish Law. In 2005, three children died in a fire in Williamsburg, Brooklyn caused when stove burners were left on during Passover. After the 2015 fire, the New York City Fire Department distributed a pamphlet titled "Fire Safety for Jewish Observances" to nearby homes. In response to the fire, many Jews in Brooklyn purchased smoke detectors before the following Sabbath.

...
Wikipedia