We accidentally cut a onion with a pareve knife on a milchige cutting board. I know the onion is milky. Is the knife also?
If so, do we need to wait till it is not a ben yomo to kasher it and reverse it back to pareve?
B’dieved we say that the knife is still pareve. If you want to kasher the knife (to do like all opinions) you do not have to wait 24hrs. since the absorbed taste is not trief, just milchig. But don’t kasher it in a fleishig pot.
See Tuv Taam Vdaas 3-215 and Daas torah 96-1, Chochmas Odom 56-2, that the cutting board can also give taste to the food through the pressure of the knife. See Ohel Yackov that R’ Eliyashev said to be machmir with this only l’chatchila. Others however disagree with the whole concept see Sefer Yehoshua siman 124, Kovetz Mbais Halevi (Yoreh Deah pg. 35) is to be maikel b’dieved if it was already done, and this seems to be the minhag of the poskim, (see Ohel Yackov 96 ftnt. 27.