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1
Preheat the oven to 425.
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2
Rub the tenderloin all over with 1 tablespoon of the olive oil and season generously with salt and pepper.
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3
In a medium roasting pan or enameled cast-iron baking dish, heat the remaining 2 tablespoons of olive oil until shimmering.
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4
Add the tenderloin to the pan and cook over moderately high heat until browned all over, about 7 minutes.
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5
Add the garlic cloves and thyme sprigs and cook for 1 minute longer.
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6
Add 1/2 cup of the chicken stock to the pan and transfer the meat to the oven.
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7
Roast the tenderloin for 5 minutes, then turn it over and add the remaining 1/2 cup of chicken stock.
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8
Cook the tenderloin for about 5 minutes longer, or until an instant-read thermometer inserted in the center registers 125.
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9
Meanwhile, in a large, deep skillet, cook the bacon over moderately high heat until it is browned and crisp, about 6 minutes.
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10
Transfer the bacon to paper towels to drain, then cut it into 1-inch pieces.
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11
Pour off all but 2 tablespoons of the bacon fat from the skillet.
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12
Add the butter and stir in the leeks.
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13
Cover the skillet with a sheet of wax paper and a tight-fitting lid and cook the leeks over moderate heat, stirring once or twice, until they are tender, about 8 minutes.
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14
Uncover the skillet, increase the heat to moderately high and cook the leeks until they are lightly browned, about 2 minutes.
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15
Add the creme fraiche, season with salt and pepper and cook over low heat until creamy and thickened, 5 to 6 minutes longer.
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16
Stir in the bacon.
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17
Transfer the roast to a cutting board and let stand for 10 minutes.
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18
Remove the strings and cut into thick slices.
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19
Serve the tenderloin with the creamed leeks, roasted garlic and thyme sprigs.
