Instructions

Adjust oven rack to lower-middle position and heat oven to 225 degrees. Melt 1 tablespoon butter in 8-inch skillet over medium-low heat. Add half of garlic and cook, stirring frequently, until golden, 5 to 7 minutes. Transfer mixture to bowl and refrigerate while preparing pork.
Microwave ¼ cup broth and porcini in covered bowl until steaming, about 1 minute. Let sit until softened, about 5 minutes. Drain mushrooms through fine-mesh strainer lined with coffee filter, reserve liquid, and chop mushrooms.
Position roast fat side up. Insert knife one-third of way up from bottom of roast along 1 long side and cut horizontally, keeping knife level with first cut and stopping ½ inch before edge. Open up flap. Keeping knife parallel to cutting board, cut through thicker half of roast, keeping knife level with first cut and stopping about ½ inch before edge. Open up this flap. If uneven, cover with plastic wrap and use meat pounder to even out. Sprinkle 1 tablespoon salt evenly over both sides of loin (½ tablespoon per side) and thoroughly rub into pork until surface is slightly tacky. Sprinkle sugar evenly over inside of loin and then spread with cooled toasted garlic mixture. Starting from short side, fold roast back together like business letter and tie with twine at 1-inch intervals. Sprinkle tied roast evenly with herbes de Provence and season with pepper.
Heat 1 tablespoon oil in large Dutch oven over medium-high heat until just smoking. Add roast, fat side down, and brown on top and sides (do not brown bottom of roast), 5 to 8 minutes. Transfer to large plate. Reduce heat to medium and add remaining tablespoon oil, cremini, and onion; cook, stirring frequently, until onion is softened and browned, 5 to 7 minutes. Stir in remaining sliced garlic and cook until fragrant, about 30 seconds. Stir in Marsala, thyme, bay leaf, and chopped porcini, and cook for 30 seconds. Return roast, fat side up, to pot; place large sheet of foil over pot and cover tightly with lid. Transfer pot to oven and cook until pork registers 140 degrees, 50 to 90 minutes (short, thick roasts will take longer than long, thin ones).
Transfer roast to carving board, tent loosely with foil, and let rest for 20 minutes. While pork rests, sprinkle gelatin over porcini soaking liquid and let sit until gelatin softens, about 5 minutes. Remove and discard thyme sprigs and bay leaf from jus. Pour jus into 2-cup measuring cup and, if necessary, add chicken broth to measure 1¼ cups. Return jus to pot and bring to simmer over medium heat. Whisk softened gelatin mixture, remaining 1 tablespoon butter, parsley, and lemon juice into jus and season with salt and pepper to taste; remove from heat and cover to keep warm. Slice pork into 1/2-inch-thick slices, adding any accumulated juices to sauce. Serve pork, passing sauce separately.