Crossword Genius

How food may be served with nothing spent from monthly allowance (2,6)

Ross

I believe the answer is:

au gratin

I'm a little stuck... Click here to teach me more about this clue!

'how food may be served with nothing spent' is the definition.
I can't tell whether this defines the answer.

'from monthly allowance' is the wordplay.
I cannot quite understand how this works, but
'from' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
'allowance' could be 'rat' (I've seen this in another clue) and 'rat' is found within the answer.
The remaining letters 'aug' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for au gratin that I've seen before include "baked with a coating of breadcrumbs" , "Covered with breadcrumbs or grated cheese" , "Like some food" , "Served with browned cheese and breadcrumbs" , "Cooked with browned melted cheese" .)

I've seen this clue in The Guardian.
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