1. Preheat oven to 425°F. Combine 1 tablespoon beer, mustard, 3 cloves garlic and parsley. Spread mixture over top surface of beef Roast.
2. Place Roast on rack in shallow roasting pan. Do not add water or cover. Roast in 425°F oven 30 to 40 minutes for medium rare; 40 to 50 minutes for medium doneness.
3. Meanwhile, heat 1 tablespoon butter in large nonstick skillet over medium heat until melted. Add mushrooms; cook and stir 4 to 5 minutes or until mushrooms begin to brown. Add remaining clove garlic; cook and stir 30 to 45 seconds or until fragrant. Add all but 2 tablespoons beef broth and remaining beer to skillet, stirring until browned bits attached to bottom of pan are dissolved. Bring to a boil; cook 13 to 15 minutes or until mixture is reduced to 1-1/2 cups. Combine remaining 2 tablespoons beef broth and cornstarch; stir into mushroom mixture. Bring to a boil; cook 1 to 2 minutes or until sauce thickens, stirring constantly. Remove from heat; add remaining 1 tablespoon butter, stirring until melted. Season with salt, as desired.
4. Remove Roast when instant-read thermometer registers 135°F for medium rare; 150°F for medium. Transfer Roast to carving board; tent loosely with aluminum foil. Let stand 20-25 minutes. (Temperature will continue to rise about 10°F to reach 145°F for medium rare; 160°F for medium.)
5. Carve Roast across the grain into thin slices. Season with salt and pepper, as desired. Serve with sauce.