Preheat oven to 350. Lightly oil a 1 3/4-quart casserole or spray with nonstick cooking spray.
Melt 1 tablespoon of the butter in a large nonstick skillet over medium heat; add barley and cook 2-3 minutes, stirring frequently until lightly browned. Place in prepared casserole.
Melt remaining butter in skillet, then add onions and cauliflower; cook, stirring frequently, 5 minutes. Add mushrooms and carrots; cook 5 more minutes, stirring frequently. Add vegetables to casserole.
In a small bowl, combine water, broth mix, garlic powder and pepper; mix well and add to casserole.
Mix well, cover and bake 1 hour and 15 minutes, until barley is tender and most of the liquid has been absorbed.
Stir several times while baking. Let stand 5 minutes, then mix and serve.