Pickled Brains

Place cauliflower in a large glass heatproof jar or canister, positioning florets against the glass; set aside.

Fill a large pot with 8 cups water. Add both vinegars, sugar, salt, peppercorns, mustard seeds, and onions. Bring to a boil.

Carefully pour boiling liquid over cauliflower; let cool to room temperature. Cover and transfer to refrigerator. Let chill overnight or up to 1 week.