Preheat oven to 350 degrees F. In a medium saucepan combine mushrooms, the water, barley, and bouillon cube. Bring to boiling; reduce heat. Simmer, covered, for 12 to 15 minutes or until barley is tender; drain.
Meanwhile, halve sweet peppers lengthwise; remove seeds and membranes. If desired, precook peppers in boiling water for 3 minutes. Invert onto paper towels to drain.
In a medium bowl combine egg, tomato, 1/2 cup of the cheese, the zucchini, bread crumbs, basil, rosemary, onion salt, and hot pepper sauce. Stir in cooked barley mixture. Place peppers, cut sides up, in an ungreased 2-quart baking dish. Spoon barley mixture into peppers.
Bake, covered, for 20 to 25 minutes or until barley mixture is heated through. Sprinkle with the remaining 1/4 cup cheese. Bake, uncovered, about 2 minutes more or until cheese is melted.