Answer by Rabbi Chaim Hillel Raskin – Rov in Kfar Chabad Beis, Israel:
Generally speaking, the transfer of taste between a food and utensil requires heat (or soaking or salting). However, the combination of pressure (duchaka d’sakina) against a sharp food (davar charif), such as radishes and onions, can transfer taste between them even without heat.
Thus, an onion cut with a fleishig knife, even if the knife isn’t ben yomo, absorbs the taste of meat. (L’chatchila, the entire onion is considered fleishig, but b’dieved we consider only a layer of k’dei netila, approx. 2 cm).
As a rule, taste absorbed in a clean utensil that wasn’t used in the past 24 hours (eino ben yomo) is considered stale (pagum) and is incapable of making food unkosher. Sharp foods are an exception to this rule, as they revitalize the taste making it fresh again.
Pareve food cooked in a clean fleishig pot that was used with meat within 24 hours (ben yomo) absorbs some of that meat flavor from the walls of the pot. However, this secondary flavor, known as nat bar nat (nosein taam bar nosein taam – “a secondary transfusion of taste”), is generally considered too “weak” to pose an issue of basar b’chalav.
This has two practical applications:
(1) Pasta cooked in a fleishig pot doesn’t cause one to become fleishig, and one may eat dairy afterwards without waiting. This is true even for sharp foods; thus, an onion fried in a clean fleishig pan or cut with a clean fleishig knife doesn’t make one fleishig to require waiting before eating dairy.
(2) Pasta cooked in a fleishig pot that was mistakenly mixed with cheese may be eaten (though l’chatchila it should not be mixed with dairy). This leniency doesn’t apply to sharp foods, since the sharpness extracts a “primary” infusion of meat from the pan or knife into the onion. Thus, if this onion was mixed with dairy, the mixture may not be eaten.
Whether an onion, or another sharp food, cut with a milchig knife may be eaten while one is still fleishig, is the subject of debate. Some are stringent regardless, some are lenient regardless, and others prohibit only when the knife was ben yomo.
In practice, an onion cut with a milchig knife may be eaten after meat. Yet, when choosing a knife for onions and other sharp foods in this situation, a knife that is not ben yomo should be used, and some use a pareve knife to satisfy even the stringent opinions.
Eating an onion cut with a fleishig knife while milchig is permissible according to all opinions, since according to strict halacha one may eat meat after milk as long as one cleans their mouth in between.
Published in the Weekly Farbrengen – Lma’an Yishme’u by Merkaz Anash. See Sources