Using את implies a definitive. English does not modify the question in such a case.
Without את the answer requested would be indefinite (An apple), with את it is definite (The "four season salad")
In the first case a definite response is acceptable, in the second an indefinite response is not acceptable.
With מי it is always definite, so you couldn't say "מי הוא אוכל?"
In English, use "who" when the person in question is the subject of the sentence (usually doing the action): - Who started the fire? - Who are you? Use "whom" when the person in question receives the action or when that person is the object of a prepositional phrase: - To whom were you speaking? - These books belong to whom? - Whom did you pinch?
Most English speakers do not know how to correctly use who/whom.
In Hebrew, I am assuming that you would use את מי in the place of "whom" and מי in the place of "who." The biggest reason it's confusing is because "whom" is almost never used in English anymore.