Taken from the Kashrus in the Kitchen Q & A by Rabbi A Weisenfeld
What is considered a sharp food?
The following foods when raw; onion, radish, garlic, leek, and horseradish, lemons, limes, esrog. Lemon juice, salty fish, olives, pickles, brine and according to some vinegar.
Preparation of sharp food : includes such objects as grates, knives, slicer, choppers, pestle,shredder and forks.Therefore one should be careful to mark these objects in a manner which is easily noticeable to know if they are meat, diary, or pareve.
Does one need to wait 6 hours between eating an onion which was cut with a clean meat knife and eating dairy? No
One should not cook a meat onion with dairy foods. If one did so and wishes to eat the food, he must make sure that the diary food is 60 times the amount as the size of the knife blade that was used to cut the onion. ( only the part that was actually used must be used in calculations.
Even if the knife was not used in the last twenty-four hours when cutting sharp foods the taste comes out due to the pressure the knife applies.
You are not allowed to cut an onion with a diary knife on meaty plate.
A peeled onion placed on a non pareve plate stays pareve. Placing a peeled onion on cold meat, makes that onion become meaty.
After eating sharp food that was fried in meaty pan, one does not need to wait six hours before eating diary.