Preheat oven to 400°.
To prepare beef, combine Beef Stock and water in a saucepan; bring to a simmer. Place beef in a 13 x 9-inch baking dish; pour stock mixture over beef. Bake at 400° for 2 hours or until beef is tender.
Remove beef from the cooking liquid; cover and keep warm. Strain cooking liquid through a sieve over a bowl. Place a zip-top bag inside a 2-cup glass measure. Pour liquid into bag; let stand 10 minutes (fat will rise to the top). Seal bag; carefully snip off 1 corner of bag. Drain liquid into a saucepan, stopping before the fat layer reaches the opening; discard fat. Bring liquid to a boil; reduce to simmer. Cook 20 minutes or until reduced to 1 cup. Remove from heat; stir in 1 teaspoon thyme, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
To prepare potatoes, place potatoes in a saucepan; cover with water. Bring to a boil. Reduce heat, and cook 12 minutes or until very tender. Drain. Return potatoes to pan. Add milk, butter, 1/2 teaspoon salt, and 1/4 teaspoon pepper; mash with a potato masher to desired consistency. Cook for 2 minutes or until thoroughly heated. Serve with beef. Serve sauce over beef; garnish with remaining 1 teaspoon thyme.