Cut onion with dairy knife and cooked it in meaty pot

If someone cut an onion with a dairy knife and cooked it in a meaty pot that hadn’t been used for at least the last 24 hours, what’s the status of the pot and what’s the status of the onion? 

As a rule, a “dairy” knife is not really dairy, since it’s generally not used to cut hot dairy products in a “first-order vessel” (“cli rishon”, a vessel that was used to heat the product). However, if you know that the knife was, in fact, used even once to cut a hot dairy product in a “first-order vessel”, for example, a pizza in its baking tray or a hot cheese casserole, then the knife is really dairy, and everything is forbidden — both the onion and the pot.

Also, if you have a dairy microwave oven at home, and you are used to cutting things that were heated in the microwave on its turning plate with this knife, that is also considered a first-order vessel, and both the onion and the pot are forbidden.

In the opposite case, when an onion was cut with a meaty knife and then cooked in a dairy pot, everything is forbidden, since it is quite common to cut hot pieces of meat inside a first-order vessel with a knife. Unless you know for a fact that this knife was never used to cut hot meat in a first-order vessel, both the onion and the pot are going to be prohibited.  

Shulchan Oruch, section Yore Dea, chapter 96