Pour 1/4 cup of Cognac, Armagnac, or Port over the prunes. Cover and refrigerate overnight. Add 1/2 teaspoon of the salt, black pepper to taste, and 1/2 teaspoon of the thyme to the prunes.

Place loin on work surface, pocket facing up. Stuff the prunes inside the pocket, so that they run along the length of the loin in a thin line. Don't let them glob up in the middle of the loin. Roll the loin and tie with kitchen string.

Rub loin generously with salt, pepper, and thyme.

Place rack in center of oven. Heat oven to 500°F.

Place loin in a roasting pan just large enough to hold it. Roast for 40 minutes, or until meat reaches an internal temperature of 145°F. The meat might still be slightly pink, but this is fine. For meat without any trace of pink, roast for 50 minutes, or until internal temperature reaches 160°F. Don't overcook the roast, or it will be dry and unappealing.

Remove roast to a platter. Snip off strings. Pour or spoon off excess fat from roasting pan. Place pan on stove over medium-high heat. Add remaining Cognac and stock, scraping pan with wooden spoon to scoop up crispy bits that will flavor sauce. Serve in a sauceboat with the roast.