“Oignon”: a spelling that reflects the errors of the French language. The word comes from the popular Latin "unio", that is to say "unity" Then "onion", which is linked to "unus", "one, only one" It was then written as follows: “unniun” in the 12th century; “oingnun’ in the 13th century.
It was only in the 14th century that we could read it as we know it today.