How to Make It
Preheat the oven to 375°. In a medium saucepan, bring the Armagnac to a boil. Add the prunes and simmer over low heat for 3 minutes. Set aside.
In a small bowl, combine the minced garlic, caraway seeds and 1 teaspoon of salt. Rub the mixture all over the boneless pork loin and let stand at room temperature for 1 hour.
Set the rack of rib bones in a large roasting pan, meaty side up, and season with salt and pepper. Set the pork loin on the rack, fat side down. Add the 6 peeled garlic cloves to the pan and pour in 2 cups of water. Roast the pork for 30 minutes. Add 1 cup of water to the pan and roast the meat for 30 minutes longer. Turn the pork loin fat side up and roast for about 30 minutes longer, or until an instant-read thermometer inserted in the thickest part of the meat registers 145°. Transfer the pork to a carving board and cover loosely with foil for at least 10 minutes or up to 20 minutes.
In a small heatproof bowl, blend the butter with the flour to make a paste. Turn the rib rack meaty side down. Set the pan over 2 burners on moderately high heat. Add 3 cups of water and simmer for 2 minutes, scraping up the browned bits from the bottom and sides of the pan. Reduce the heat to moderate and simmer for 10 minutes, stirring often and mashing the garlic cloves.
Whisk 1/2 cup of the pan juices into the butter paste until smooth. Remove the rib rack from the roasting pan. Whisk the dissolved butter paste into the pan juices and simmer, whisking often, for 5 minutes. Strain the sauce into a medium saucepan. Add the prunes and simmer over moderate heat for 5 minutes. Season with salt and pepper. Carve the pork loin into 1/4-inch thick slices and serve with the sauce.