Standing Rib Roast with Potatoes and Portabello Mushrooms

Preheat oven to 475F. In a small bowl stir together first seven 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 1 tablespoon seasoning mixture and rub remainder on top and sides of beef. Roast beef in lower third of oven 20 minutes. Cut potatoes 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 350F. 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 1300F. for medium-rare. While beef, carrots 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. Transfer beef to a platter and let rest, covered loosely with foil, 20 to 30 minutes. Transfer potatoes to another platter and keep warm. With bulb baster or spoon skim all but about 1/4 cup fat from pan. In pan on top of stove saute 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 manie and boil, stirring, about 3 minutes, or until thickened slightly.