Translating efter as after shouldn't be wrong here. It's an unusual context to 'ask after something' normally that is in the context of another persons wellbeing rather than a menu, but its not actually wrong, just a more literal translation and a slightly odder way of speech.
Prepositions shouldn't really be learnt as "X" in one language meaning "Y" in another, they should be learnt in context. "Efter" here is used with "spørger" like "for" is used with "ask" in order to request an item. Translated as the two words working together rather than as two separate words.