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1
In a roasting pan toss the onions and the garlic with the oil, add the pork loin, and season it with salt and pepper.
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2
Rub the coriander seeds over the pork and roast the pork in the middle of a preheated 350F.
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3
oven for 1 to 1 1/4 hours, or until a meat thermometer registers 155F.
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4
to 160F.
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5
While the pork is roasting, remove 3 or 4 long strips of zest from 1 of the grapefruit with a vegetable peeler, scrape off as much of the white pith from the zest as possible, and cut enough of the zest into fine julienne strips to measure 1/4 cup.
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6
Cut away the zest, pith, and membranes from all 3 grapefruit with a serrated knife and, working over a bowl to catch the juice, cut the flesh into sections, reserving the juice.
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7
Transfer the sections to a sieve and let them drain.
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8
In a small heavy saucepan combine 3 tablespoons of the sugar, the zest strips, and 1/2 cup of the reserved juice, reserving the remaining juice for another use.
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9
Bring the mixture to a boil, stirring until the sugar is dissolved, and boil it until it is thickened and turns a light caramel.
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10
Transfer the pork to a cutting board and let it stand, covered loosely with foil, for 10 minutes.
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11
While the pork is standing, deglaze the roasting pan with the wine, boiling the mixture and scraping up the brown bits, until the wine is reduced almost completely and add the broth.
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12
Pour the deglazing mixture through a fine sieve into the pan containing zest mixture, pressing hard on the solids, and bring the gravy to a boil, stirring.
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13
Add the beurre manie, a little at a time, whisking until the gravy is smooth, simmer the gravy, whisking occasionally, for 2 minutes, and whisk in any juices that have accumulated on the cutting board.
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14
Season the gravy with salt and pepper and keep it warm.
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15
Arrange the grapefruit sections in a shallow baking pan or gratin dish just large enough to hold them in one layer, sprinkle them with the remaining 1 1/2 tablespoons sugar, and broil them under a preheated broiler about 2 to 3 inches from the heat for 3 to 5 minutes, or until they are just golden.
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16
Discard the string from the pork, cut the pork into 1/2-inch-thick slices, and serve it with the gravy and the grapefruit sections.
