Spinach- and Mushroom-Stuffed Tenderloin

Prepare Spinach-Mushroom Stuffing: In 12-inch skillet, heat 4 tablespoons margarine or butter (1/2 stick) over medium-high heat; add 1 pound mushrooms, coarsely chopped, and cook until golden and liquid evaporates, 12 to 15 minutes. Stir in 2 tablespoons dry vermouth; cook 1 minute longer. Remove skillet from heat; stir in 1 package (10 ounces) frozen chopped spinach, thawed and squeezed dry, 2 tablespoons grated Parmesan cheese, 2 tablespoons plain dried bread crumbs, 1 teaspoon chopped fresh thyme leaves, 1/4 teaspoon salt, and 1/4 teaspoon coarsely ground black pepper. Prepare Tenderloin: In cup, mix thyme, salt, and pepper; rub mixture all over tenderloin. Turn thinner end under tenderloin to make meat an even thickness. With sharp knife, cut 1 1/2-inch-deep slit in tenderloin, starting 2 inches from thicker end and ending 2 inches from thinner end. Preheat oven to 425 degrees F. Spoon Spinach-Mushroom Stuffing into slit in tenderloin. With string, tie tenderloin at 2-inch intervals to help hold its shape. Place stuffed tenderloin on rack in large roasting pan (17-inch by 11 1/2-inch); roast tenderloin 30 minutes. Meanwhile, in small saucepan, melt 1 tablespoon margarine or butter over low heat. Remove saucepan from heat; stir in bread crumbs. Set bread-crumb topping aside. Remove tenderloin from oven; sprinkle bread-crumb topping over stuffing. Roast tenderloin 10 to 15 minutes longer, until bread-crumb topping is golden and meat thermometer reaches 140 degrees F. Internal temperature of meat will rise to 145 degrees F (medium-rare) upon standing. Or, roast to desired doneness. Transfer tenderloin to large platter; let stand 10 minutes for easier slicing. Prepare Mushroom Gravy: While tenderloin is standing, add vermouth and 1/2 cup chicken broth to drippings in roasting pan and stir over low heat until browned bits are loosened from pan. Pour drippings mixture into 4-cup measuring cup; let stand a few seconds until fat separates from meat juice. Skim fat from meat-juice mixture; add remaining chicken broth and enough water to equal 2 1/2 cups; set aside. In 12-inch skillet, heat remaining 1 tablespoon margarine or butter over medium-high heat. Add mushrooms and cook until golden and liquid evaporates, about 12 minutes. Stir in flour. Gradually stir meat-juice mixture into mushrooms and cook, stirring constantly, until gravy boils and thickens slightly; boil 1 minute. To serve, remove string. Cut stuffed tenderloin into slices; garnish with thyme sprigs. Serve with mushroom gravy.