。
At lunch, mother served me what I wanted to eat.
Literal
Mother [topic-は] noon [at-に] I [subject-が] want-to-eat thing [obj-を] served-for-me.
Two things to notice. First, 私が食べたいもの is a relative clause modifying もの — in Japanese, modifiers precede the noun, so the whole clause sits before もの. Second, the subject of 食べたい inside that clause is marked with が, not は, because it's inside a subordinate clause (は prefers main clauses). ~てくれる marks the benefit direction. お昼 is the politer / more colloquial way of saying 'lunch' / 'noon,' with お prefix.