Adapted from mrfood.com
In a large resealable plastic bag, combine oil, onion, and garlic; close tightly and shake to mix well. Place roast in bag, seal, and turn to coat completely with mixture. Marinate 2 hours in refrigerator. Preheat oven to 350 degrees F. Line a roasting pan with aluminum foil. In a medium bowl, combine salt, pepper, and water; mix well. Place half the salt mixture in center of prepared pan. Place roast over salt mixture then coat with remaining salt mixture, completely coating roast and forming a crust about 1/2 inch thick. Bake 55 minutes for medium doneness, or until desired doneness beyond that. Remove from oven and discard salt crust. Slice and serve.