。
She asked how to cook the fish.
Literal
She [topic-は] that fish's cooking-method [object-を] asked.
尋ねる is a slightly more formal cousin of 聞く ('ask'). 料理方法 is a transparent Sino-Japanese compound — 料理 ('cooking') + 方法 ('method') — Japanese stacks these two-kanji nouns freely to coin precise descriptive labels. Compare 使用方法 ('how to use'), 連絡方法 ('how to contact'). The の links 魚 to 料理方法 as 'the fish's cooking-method', i.e. 'how to cook the fish'. The を marks that compound noun — the question itself — as the thing being asked about.