Standing Rib Roast with Potatoes and Portobello Mushrooms

1 tablespoon chopped fresh rosemary leaves 1 tablespoon chopped fresh thyme leaves 2 garlic cloves, chopped 1 1/2 teaspoons coarse salt 1 teaspoon freshly ground black pepper a 3-rib standing beef rib roast (about 8 1/2 pounds), all but 1/4-inch layer of fat removed 1 tablespoon vegetable oil 5 pounds russet (baking) potatoes (about 10) 1 1/2 pounds Portobello mushrooms, wiped clean 1 3/4 cups beef broth (a 15 1/2-ounce can) 1 cup water a beurre manié made by kneading together 1 tablespoon softened unsalted butter and 1 tablespoon all-purpose flour Garnish: chopped fresh flat-leafed parsley leaves (wash and dry before chopping)