If one cooks cheese (Grilled cheese) in a meat pan, does it make the cheese not kosher, and therefore a sin? List sources please.
The cheese does become not kosher and it would be a sin according to Torah for a Jew to eat it, IF it falls under scenario #2 below, but not scenario #1 below; as follows:
(We are assuming the OP is talking about an accident. Of course, we do not use meat pans for making grilled cheese.)
The first classic question is if the meat pan was actually used for hot meat within the past 24 hours? Also, was it clean?
1) If it was not used in the past 24 hours, and is currently clean, then the old taste of meat within the surfaces of the pan is called "nosen taam lifgam" or "a spoiled taste". This kind of meat taste cannot cause the grilled cheese to become forbidden. (see Shulchan Aruch Yoreh Deah 103:5)
The pan must be kashered. (S.A. Y.D. 93:1)
R' Yosef Karo (Sefardi custom) says there that meanwhile, the pan may be used for pareve (neither milk nor meat) foods. However, the Rema (94:5) and the Shach (93:3) forbid this by Ashkenazi custom.
2) If the pan was in fact used for cooking meat in the past 24 hours, then both the grilled cheese (absorbing potent meat flavor) and pan are forbidden. The food is thrown out and the pan is kashered. If the pan actually contained real meat residue, then the grilled cheese touching real meat is certainly a problem. This can happen more often with disposable pans.
BTW, I do not remember the source, but if you do not know when you last used the pan then you may assume it was longer ago than 24 hours.
Please use this as a general learning about the Halachic issues. There are many other issues that may come up and new cases might not be comparable. If you have a real question, consult your local Orthodox Rabbi.