hi! i started slicing a onion with a fleishig knife and by mistake continued with a milchig knife, whats the status of the onion? the milchig knife?
and does the same apply for dicing an onion?
Before answering your question, the poskim advise to get into the habit of only using a pareve knife when cutting onion and other sharp vegetables, for this very reason.
Regarding your question, the part of the onion that was cut with the fleishig knife is ok but the other part of the onion is controversial therefore it should not be used, this is because according to numerous poskim, the onion itself has the absorbed taste of both the meat and the dairy. Regarding the knife, based on a number of factors it would not need kashering, (However there would be a benefit to actually kasher it, and it is very easy. Just clean off the blade and stick the blade into a pot of boiling water). To say that the knife became trief we need that both knives were used for kli rishon meat and dairy, which is not so common. Aside from this there are opinions that with an aino ben yomo it is permitted.
I don’t see why dicing is different than slicing, since both forms of cutting can impart taste.
Teshuvos Vhanhagos 1- 433. Pri Megadim O:CH 447-13, D:T Y:D 96- 3,5, Chavas Daas 96-6b, Pischei Halacha pg. 327, Horav Y. Cahen shlit”a