Remove and discard root, tough outer leaves, and top from leek. Finely chop leek.
Coat a large nonstick skillet with cooking spray; add oil. Place over medium heat until hot. Add chopped leek, mushrooms, and garlic; saute until mushrooms are tender. Add barley and next 3 ingredients; bring to a boil. Cover, reduce heat, and simmer 16 to 18 minutes or until barley is tender and most of liquid is absorbed. Remove from heat; let stand 5 minutes. Sprinkle with cheese, and serve immediately.