I used my Pareve oven to bake Milchig cookies. It’s nothing that splatters onto the oven or anything that would make the oven dirty, it was just baked on a disposable baking sheet. To use the oven for Pareve again, do I need to kosher it? If yes, how so?
The oven needs to be Kashered if you want it to be Pareve, (even if none of the Milchig cookies spilled or splattered).
To Kasher the oven in this instance it is enough to heat it up on the highest setting for about half hour.
In addition, if possible, it is best not to use the oven for 24 hours. If you need to use the oven though before that than this step is not necessary.
See below for a brief discussion on the topic.
אע”פ שלא נשפך וכו’ מ”מ צריך הכשר מצד הזיעה (בנדו”ד מכיון שאסור לגרום נ”ט בר נ”ט לכתחילה כשאופה פרווה ע”מ לאכלו עם מין השני). ומש”כ לחכות מעל”ע הוא מכיון שיש מפקפקים בהכשר הזאת (ואף שי”א שאין לגרום נ”ט בר נ”ט לכתחילה אפילו אם אינו ב”י
A brief discussion on the topic:
If I bake Pareve in a Fleishig oven, is it still considered Pareve?
A Fleishig (meaty) oven affects the status of Pareve (neutral, i.e. containing no meat, poultry or dairy products) food by transferring the taste of food residue that may be on the oven rack, or which was previously absorbed through cooking splatters and steam.
Even if care was taken while cooking meat or chicken dishes to use a pan that prevents the racks and oven from being splattered, we may not mix the Pareve food with Milchigs Lechatchilah (primarily, i.e. intentionally) on account of steam from the Fleishig food that was absorbed in the walls of the oven and later released while cooking the Pareve food. (This requirement is specific to Ashkenazim. Among Sefardim, the Psak—ruling—is more lenient.)
However, if the Pareve food was mixed with Milchigs (dairy), the food may be eaten B’dieved (ex post facto), provided that the Fleishig oven was clean.
What if I want to eat my Pareve food with Milchigs? In order to bake Pareve food in a Fleishig oven with the intent of eating it with Milchigs—for example, cake with coffee or Challah at a dairy meal—the oven has to be purged of its Fleishig status. To do so in an expedient (if lenient) manner, it is enough to turn on the empty, clean oven for a short while—such as when preheating it—before baking the pareve food. There are a number of factors that would allow for this Kulah (leniency), among them the assumption that the oven originally became Fleishig through steam only. This leniency, however, does not apply to baking actual dairy food in a Fleishig oven.
(The Halachos of a Fleishig oven that has not been used for over twenty-four hours—Eino Ben Yomo–are not as strict).