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Preheat oven to 400°F
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Cut top 1/2 inch off heads of garlic, exposing cloves.
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Place each on square of foil.
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Drizzle with oil, then enclose in foil.
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Place on oven rack; bake until garlic is tender, about 45 minutes.
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Open foil; cool garlic slightly.
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Hold 1 head of garlic, cut side down, over medium bowl and squeeze, releasing roasted garlic pulp into bowl.
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Repeat with remaining garlic.
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Mash until smooth.
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Whisk in mayonnaise, buttermilk, Parmesan cheese, lemon juice, and Worcestershire sauce.
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Season to taste with salt and pepper.
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Cover and chill until ready to use, up to 2 days.