Whisk garlic, mayonnaise, sour cream, vinegar and sugar in a large bowl. Add broccoli, water chestnuts, bacon, cranberries and pepper; stir to coat with the dressing.
Most supermarkets sell broccoli crowns, which are the tops of the bunches, with the stalks cut off. Although crowns are more expensive than entire bunches, they are convenient and there is considerably less waste.
To Make Ahead: Cover and refrigerate for up to 1 day.
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