In 12-inch skillet, cook onion in butter over medium-high heat 3 minutes or until tender. Add garlic; cook and stir 30 seconds. Add mushrooms, thyme, rosemary and 1/2 teaspoon of the salt. Cook 5 minutes or until mushrooms are tender. Stir in vinegar; cook 2 minutes or until liquid has evaporated. Cool. Spoon mushroom mixture over beef to within 1/2 inch of edges. Roll up beef, starting with long side, and tie with kitchen string at 2-inch intervals. Rub with pepper and remaining 1/2 teaspoon salt. Place seam side down on rack in roasting pan. Insert ovenproof meat thermometer so tip is in thickest part of beef.