In 2-quart nonstick saucepan, heat broth, water, thyme and salt to boiling. Stir in mushrooms, orzo and barley. Return to boiling.
Reduce heat to low; cover and simmer 15 to 18 minutes or until orzo and barley are tender and liquid is absorbed.
Stir in onions and lemon peel. If desired, season to taste with pepper.