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1
Preheat oven to 475F.
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2
Put roast, ribs side down, in center of a 13- by 9-inch flameproof roasting pan and rub all over with 1/2 tablespoon oil.
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3
Cut about 1/2 inch from tops of garlic heads to expose cloves and discard tops.
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4
Rub beef all over with cut sides of garlic and sprinkle with 2 tablespoons thyme, pressing to adhere.
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5
Season beef generously with salt and pepper.
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6
Put garlic heads on a double layer of foil and drizzle with remaining 1/2 tablespoon oil.
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7
Wrap garlic tightly in foil.
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8
Roast beef in middle of oven 30 minutes.
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9
Remove beef from oven and with a bulb baster skim all but about 1/2 cup fat from pan.
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10
Baste beef with fat remaining in pan.
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11
Reduce temperature to 375F.
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12
Add garlic to oven.
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13
Roast beef, with garlic, 1 1/4 to 1 1/2 hours more, or until a meat thermometer inserted into center of meat registers 115F.
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14
Transfer garlic in foil to a rack and transfer beef to a platter, reserving pan juices.
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15
Let beef stand 25 minutes.
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16
(Meat will continue to cook, reaching about 130F.
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17
for medium-rare.)
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18
While beef is standing, remove all but about 2 tablespoons fat from pan.
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19
Add shallot to pan and on top of stove cook over moderate heat, stirring, 2 minutes.
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20
Unwrap garlic.
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21
Protecting your hand with a doubled paper towel, squeeze roasted garlic into shallot mixture and discard skins.
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22
Add vinegar, wine, bay leaf, and remaining tablespoon thyme and boil mixture, scraping up brown bits and mashing garlic with a wooden spatula against bottom of pan, until reduced to about 1/2 cup, about 5 minutes.
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23
If using demiglace, stir into garlic mixture and boil sauce until reduced to about 2 cups, about 5 minutes.
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24
Whisk in butter.
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25
If using beef broth, in a small bowl mash together butter and flour to make a beurre manie.
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26
Stir beef broth into garlic mixture and boil sauce until reduced to about 2 cups.
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27
Whisk beurre manie into broth mixture and cook over moderate heat, whisking, until sauce is smooth and slightly thickened, about 3 minutes.
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28
Pour sauce through a fine sieve into a bowl, pressing hard on solids.
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29
Season sauce with salt and pepper.
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30
Cut rib roast into slices and serve with sauce.
