PREPARATION:

1. Bring water to a boil in 1-quart saucepan. Add barley and 1/4 teaspoon salt. Cover; reduce heat and simmer 45 minutes or until barley is tender and most of the water has evaporated. Let stand covered, 5 minutes.
2. Preheat oven to 375°F. Spray medium skillet with cooking spray. Cook onion, zucchini and bell pepper over medium-low heat about 10 minutes or until soft. Stir in flour, remaining 1/4 teaspoon salt; cook 1 to 2 minutes. Add milk, stirring constantly; cook and stir until slightly thickened. Remove from heat. Add barley, 3/4 cup cheese and mustard; stir until cheese is melted. Season to taste with seasoned pepper.
3. Spread in even layer in casserole. Sprinkle with remaining 1/4 cup cheese. Bake 20 minutes or until hot. Preheat broiler. Broil casserole 1 to 2 minutes or until cheese is lightly browned.