I purchase a used metal slow cooker which uses Corningware inserts. I have a brand new Corningware insert, so that is not a kashrus concern. Do I need to Kasher the metal slow cooker itself, and if so, how?

ב"ה Shalom It's a good thing you brought this question to light, because I assume others have encountered the same issue. On the one hand, when two dry hot utensils touch each one meat and one dairy for example, they do not become not kosher because no transferring of meat or milk took place. So it would seem on the one hand, perhaps the cooker itself need not be kashered. However, there is sometimes spillage to the inner pot. Also when the Corning ware is covered, steam from the pot may enter the inner pot (though technically I am not certain about this) and in addition you purchased a slow cooker, which in most likelihood was used for absolute "treif" ,therefore I think the inner pot should be kashered as well. This is achieved without water but by "libun kal" which means leaving the heat element in the inner pot on until it reaches a heat when placing some straw like material it would begin to burn. Once that is done you can use your slow cooker with the new insert and cover. All the best