Bring 4 cups water to a boil in a large saucepan. Add barley; cover, reduce heat, and simmer for 45 minutes.
Remove from heat, and let stand, covered, 5 minutes.
Preheat oven to 350°.
Remove and discard stems from mushrooms, and slice mushroom caps. Heat a large nonstick skillet coated with cooking spray over medium-high heat until hot. Add mushrooms, and sauté 3 minutes. Remove from skillet. Heat oil in skillet over medium heat. Stir in onion; cover and cook 6 minutes. Add bell peppers; cover and cook 2 minutes. Stir in the cooked barley, mushrooms, broth, and next 4 ingredients (broth through black pepper).
Spoon half of barley mixture into an 11 x 7-inch baking dish coated with cooking spray, and sprinkle with half of cheese. Repeat layers. Cover and bake at 350° for 45 minutes.