[mdp2021]

You are misreading it completely: it is a preliminary "Do you slice the truffle on it? How is the dish prepared?". There exist perfectly appropriate ways of "«insisting»" ("Thank you, good, I would like to have it sliced in presence").

You put the slices on top: they are not dishes with preliminary preparation (which imply mixing the ingredients). It is just normal to do it in presence - and definitely sensible.