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She knows everything about cooking.
Literal
She [topic-は] cooking [possessive-の] thing [conditional-なら] anything knows.
~のことならなんでも知っている means 'knows everything about X.' なら marks a conditional/topic — 'if it's about cooking.' なんでも ('anything, everything') combines 何 with でも. The combination のことなら creates a natural topic frame: 'when it comes to matters of cooking.'