How to Make It
Preheat the oven to 425°. Rub the tenderloin all over with 1 tablespoon of the olive oil and season generously with salt and pepper.
In a medium roasting pan or enameled cast-iron baking dish, heat the remaining 2 tablespoons of olive oil until shimmering. Add the tenderloin to the pan and cook over moderately high heat until browned all over, about 7 minutes. Add the garlic cloves and thyme sprigs and cook for 1 minute longer. Add 1/2 cup of the chicken stock to the pan and transfer the meat to the oven. Roast the tenderloin for 5 minutes, then turn it over and add the remaining 1/2 cup of chicken stock. Cook the tenderloin for about 5 minutes longer, or until an instant-read thermometer inserted in the center registers 125°.
Meanwhile, in a large, deep skillet, cook the bacon over moderately high heat until it is browned and crisp, about 6 minutes. Transfer the bacon to paper towels to drain, then cut it into 1-inch pieces.
Pour off all but 2 tablespoons of the bacon fat from the skillet. Add the butter and stir in the leeks. 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. Uncover the skillet, increase the heat to moderately high and cook the leeks until they are lightly browned, about 2 minutes. Add the crème fraîche, season with salt and pepper and cook over low heat until creamy and thickened, 5 to 6 minutes longer. Stir in the bacon.
Transfer the roast to a cutting board and let stand for 10 minutes. Remove the strings and cut into thick slices. Serve the tenderloin with the creamed leeks, roasted garlic and thyme sprigs.