really depends how it’s setup in the first place
if you have something (in this case Onions) preselected, the customer can just de-select it
if it is only in your description that an item has onions, then of course th eplugin will not know anything about it
however, you could simply add a “No Onion” topping so to speak a customer can then select (you could also set negative prices
there if you wanted!)

Naturally, the customer can of course also write “no onions” into a textbox