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Put oven rack in middle position and preheat oven to 425.
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Cut off and discard 1/2 inch tops of garlic heads, exposing cloves.
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Put garlic in pie plate with oil, herbs, salt, and pepper and cover plate tightly with a double layer of foil.
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Roast until garlic is golden and tender, 1-1 1/4 hours.
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When garlic is cool enough to handle, squeeze cloves into a bowl and pour oil through a fine-mesh sieve onto garlic.
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Mash well with a fork and season with salt.