Ingredients

Serves 8

　　

For seasoning mixture

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)

　　

Step 1

Preheat oven to 475°F.

　　

Step 2

Make seasoning mixture:

　　

Step 3

In a small bowl stir together seasoning mixture ingredients. Put roast, ribs side down, in center of a flameproof roasting pan, 18 by 12 by 2 inches, and rub oil on top and sides of beef so herbs will adhere. Reserve

　　

Step 4

1 tablespoon seasoning mixture and rub remainder on top and sides of beef. Roast beef in lower third of oven 20 minutes.

　　

Step 5

Peel potatoes and halve crosswise. Remove pan from oven and skim all but about 1/2 cup fat from pan with a bulb baster or spoon. Arrange potatoes around beef and turn with tongs to coat with fat. Season potatoes with salt and pepper. Reduce heat to 350°F. and roast beef and potatoes, turning potatoes occasionally to brown evenly, 1 1/2 hours, or until a meat thermometer inserted in fleshy part of beef registers 130°F. for medium-rare.

　　

Step 6

While beef and potatoes are roasting, separate mushroom stems and caps. Slice caps 1/3-inch thick. In a food processor pulse stems until chopped coarse and in a small saucepan combine with broth and water. Simmer mixture 10 minutes. Pour mixture through a sieve into a bowl, pressing hard on solids, and discard solids.

　　

Step 7

Transfer beef to a platter and let rest, covered loosely with foil, 20 to 30 minutes. Transfer potatoes to another platter and keep warm.

　　

Step 8

With bulb baster or spoon skim all but about 1/4 cup fat from pan. In pan on top of stove sauté mushroom caps over moderately high heat, stirring, 2 minutes. Add reserved tablespoon seasoning mixture, mushroom liquid, and any juices from beef and boil 5 minutes. Stir in beurre manié and boil, stirring, about 3 minutes, or until thickened slightly.

　　

Step 9

Serve beef with sauce and sprinkle potatoes with parsley.