בשר בחלב

(19) The choice first fruits of your soil you shall bring to the house of the LORD your God. You shall not boil a kid in its mother’s milk.

(1) It is written in the Torah: "you will not cook a kid in the milk of its mother" three times (Exodus 23:19; Exodus 34:26; Deuteronomy 14:21); once for the prohibition of cooking, once for the prohibition of eating, and once for the prohibition of receiving benefit [from the cooked meat and milk products]. The prohibition of eating is presented in the language of cooking, to say that there is no prohibition from the Torah [in regard to meat and milk] unless it is in a manner of cooking, but rabbinically it [the mixture of meat and milk] is forbidden in every way.

But if one cooks the meat of a clean animal with the milk of an unclean animal, or the meat of an unclean animal with the milk of a clean animal, he is permitted to do so and derive a benefit from it as well; punishment is incurred for eating it, but not by reason of its being a meat-milk mixture. So too, the meat of beast or bird cooked with the milk of beasts or cattle is biblically not prohibited from eating; hence, it is permissible to cook it and to derive a benefit from it. It is, however, prohibited from eating it on rabbinic grounds, so that the people may not reach out beyond what is permitted and transgress the biblical meat-milk prohibition by eating the meat of a clean animal with the milk of a clean animal, since the literal meaning of the verse is restricted to a kid in its mother's milk only. For this reason, the sages have prohibited all meat with milk.

All meat and milk [mixtures] that are not forbidden from the Torah are permitted to benefit from.

Abayei said: It's a preventative measure, lest you bring it up in a boiling tray. Either way, it would be a secondary [hot] vessel, and a secondary vessel does not cook. Rather, It's a preventative measure, lest you bring it up in a [boiling] primary cooking vessel.

In the case of a primary vessel (meaning a vessel which is used on the fire) even after it has been removed from the fire it can cook items placed in it, as long as the pot is less than Yad Soledet Bo.