Preheat 300°F. Rub ½ tsp. salt, ½ tsp. pepper, 2 tsp. garlic powder over 3 lbs. pork shoulder. Add 1 chopped onion into bottom of a heavy pot. Place pork on onions. Pour 1 bottle BBQ sauce and 1 can Dr Pepper over pork, sprinkle on 1 Tbsp. brown sugar. Cover and cook 6 hours, turning roast halfway through cooking. Once done, pull pork apart with forks. Serve on slider buns; top with pickle.