1. Preheat oven to 425°F. Using a sharp knife, remove any excess fat from loin, leaving a 1/4-inch layer of fat on top. Use a long, thin, sharp knife to pierce a hole in center of both ends of loin. Working from both ends, use handle of a large wooden spoon to force a cavity all the way through center of loin. Remove spoon and push handle through several more times, widening the cavity. Working with a few prunes, push them into the cavity with your fingers and spoon handle. Work from both ends until cavity is stuffed from end to end.
2. Place garlic, 1/2 tsp. salt, lemon juice and thyme on a cutting board. Mince into a paste. Rub paste all over stuffed loin, place fat side up in a roasting pan and pour 1/2 cup water around. Roast for 20 minutes. Reduce oven temperature to 325°F. Continue to roast, adding water to pan if necessary to prevent scorching, until a meat thermometer inserted in center of meat reads 145°F, 40 to 50 minutes. Remove to a cutting board and cover loosely with foil.
3. Skim fat from juices in roasting pan and place pan across 2 burners on the stove over medium heat. Pour in broth and bring to a boil, scraping up any browned bits on bottom of pan. Boil until reduced by about half and slightly thickened, 5 to 10 minutes. Season with salt and pepper.
4. Slice roast and serve warm with pan juices.