Please be aware that there are diverse minhagim in different communities regarding the use of ovens for various types of foods. What we write here is not intended to delegitimize any ruling you may have received from a competent rabbinic authority.
We start with the case where you use pareve ingredients for a cake baked in a fleishig pan. Does the fleishig taste, which the pan absorbed, get transferred yet again to the pareve food and turn it into fleishig? This double-removed potential taste, known as nat bar nat, is the subject of a far-reaching machloket between the Shulchan Aruch and the Rama.[1] The Rama’s ruling, accepted by Ashkenazic Jewry, is that it is proper to treat the formerly pareve food as fleishig in regard to not eating it with milchig food.
However, if the pan has not been used for twenty-four hours, then the taste remaining in its walls is considered “spoiled” and is not halachically significant. It is true that Chazal did not allow us to use non-kosher utensils even after they have remained unused for twenty-four hours. However, since, in this case, even within twenty-four hours, the fleishig status of the food is the subject of the aforementioned machloket, the cake baked in such a pan (after twenty-four hours) is considered pareve. For this reason, the Gra[2] permitted the use of such a pan for the purpose of eating it with milk. On the other hand, many Acharonim subscribe to the opinion of the Chochmat Adam,[3] with which the Rama[4] mildly implies that he agrees, that one should not set up such a situation l’chatchila.[5] In other words, if one planned to eat the cake with fleishig or pareve, and then a situation arose where he decided to eat it with milchig, he may do so. However, he should not bake with the intention to eat the cake with milk. Thus, the proper thing, based upon the above, is to use a pareve or disposable pan.
Using a fleishig oven, however, still presents halachic challenges. The Rama[6] rules that, when two foods were baked or roasted in an oven at different times, taste is transferred from one food to the other only if there was condensation (zei’ah) from both foods on the walls of the oven. A “fleishig oven,” presumably, had fleishig condensation at some time during its use. However, it is unclear how liquid a food and how insulated an oven have to be in order that there be zei’ah to transfer the fleishig taste from the walls to the food.[7] Bread and relatively dry cake batter probably do not create zei’ah in a normal oven and will remain pareve. (One must make sure that the pan does not touch a surface with fleishig residue on it.) However, a liquid batter may create zei’ah. Practical rulings on the matter depend not only on the case’s details but also on the approach of one’s rabbi. If zei’ah is present, it compromises the pareve status of both the cake and the pan. (If the oven was well-cleaned and had not been used within twenty-four hours, the pan would not need to be kashered and the cake remains pareve).
There are at least two good solutions to the problem of zei’ah in an oven. One solution is to cover the cake batter (where feasible) so that escaping moisture is insufficient to transfer taste.[8] The other solution is to do libun kal[9] on the oven before baking the cake to remove the fleishig taste from the walls and to burn any surface residue. A half-hour of heating at the oven’s highest temperature is usually sufficient to accomplish libun kal. (More time is needed if there was significant spillage that one did not remove prior to heating.) Even one who relies upon the aforementioned Gra must ensure that there is no edible residue on the walls of the oven that zei’ah could transfer to the food. If there are tiny quantities of edible residue on the wall, they will not make the food fleishig b’di’eved,[10] [11] but, with direct contact, they may affect the status of the pan.