Onion in a mixture is not the same as onion on top
The idea
Onion can be part of a cooked mixture or a fresh garnish. Those roles give different textures and strengths of flavor. Removing a visible onion topping may not remove onion from the filling or sauce underneath, which is why a complete ingredient conversation matters.
A practical next step
Tell the team whether your question is about a garnish, a preference or avoiding onion throughout the whole preparation.