Standing Rib Roast with Potatoes and Portobello Mushrooms

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

1. 01Preheat oven to 475°F.
2. 02Make seasoning mixture:
3. 03In 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
4. 041 tablespoon seasoning mixture and rub remainder on top and sides of beef. Roast beef in lower third of oven 20 minutes.
5. 05Peel 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.
6. 06While 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.
7. 07

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