For the brine: Combine the salt, brown sugar, peppercorns, coriander seeds, and 2 cups warm water in a large bowl. Stir until the salt dissolves. Add 6 cups cold water. Add the pork, cover, and refrigerate overnight (the pork should be submerged in the liquid).
Preheat the oven to 400 degrees F.
For the herb rub: Mix the garlic, olive oil, salt, pepper, thyme, and rosemary in a small bowl. Remove the pork from the brine and pat it dry (discard the brine). Spread the herb mixture over the pork loin, making sure you coat all sides of the loin.
For the pork: Heat the olive oil in a medium, heavy saute pan over high heat. Add the pork and sear until browned, 4 minutes. Carefully turn the pork over and sear until browned, another 4 minutes. Meanwhile, add the prunes and pearl onions to a 10 by 7 by 2-inch glass baking dish, creating a bed for the loin. Transfer the seared pork loin to the baking dish (making sure the loin fits in the baking dish, leaving a 1-inch border on every side).
Add the wine to the same saute pan used to sear the pork and cook, scraping the bottom of the pan to remove the browned bits, until almost evaporated, about1 minute. Stir in the pineapple juice and remove from the heat. Season with salt and pepper. Pour the mixture over the pork. Season the prunes and onions with salt and pepper.
Place the pork in the oven and roast until a thermometer inserted into the center registers 160 degrees F, about 1 hour, basting with the pan juices every 20 minutes. Transfer the pork to a cutting board, tent with foil, and let it stand for 10 minutes. Slice the pork into 1/4 to 1/2-inch thick slices and arrange on a platter. Top the pork slices with the pearl onions, prunes, and sauce.
Cook's Note: In place of a 3 1/2-pound pork loin, you can use 2 (1 1/4-pound) pork tenderloins. The total cooking time will be about 1 hour.