Directions

1. With the tip of a sharp knife, randomly poke 6 holes 1 inch deep into roast. Insert a garlic half in each hole.
2. In a deep bowl or sealable bag, combine vinegar, 1 tablespoon chopped peppercorns and rosemary. Add roast, roll around to coat, cover bowl or seal bag. Marinate in refrigerator 12 to 36 hours, turning occasionally.
3. Remove roast. Preheat oven to 300 degrees F.
4. In a skillet, over medium heat, brown meat on all sides in remaining 1 tablespoon oil. Place meat on a large square of heavy foil. Place on baking sheet. Add water to skillet, stir to loosen bits from bottom of pan. Bring up the sides of foil around meat. Pour water from skillet over meat. Toss in onions and mushrooms. Lightly season with salt & pepper. Close foil, sealing all edges with a secure double fold.
5. Roast three hours.
6. Remove roast and vegetables from foil. Cover loosely to warm. Gently boil liquid over medium heat until reduced to about 1 cup. Stir together water & flour until smooth. Stir into liquid. Cook 2 minutes. Stir in sherry and 1 teaspoon peppercorns. Thinly slice meat. Drizzle with gravy.