"to know one's onion Is this correct phrase, or maybe not?"
I think you will find that the common usage is "to know ones onions", as in "He really knows his onions!" (he knows his trade/subject). The phrase is especially used a lot in the North of England. I think you will find the Northern opposite to this phrase must be "He doesn't know his *** from his elbow." (He hasn't got a clue).
BR MOTU