。
Mom fixed us lunch.
Literal
Mom [subject-が] lunch [gen-の] preparation [obj-を] did-for-us.
Notice が rather than は on 母さん — this introduces new information (who did the action), rather than topicalizing her. The pattern 'noun+の+用意をする' ('do the preparation of X') is a common way to express 'to prepare X,' more formal-sounding than simply 用意する. ~てくれた frames the favor as flowing to the speaker's side.