1. In a large nonstick skillet, heat oil over medium heat. Add onion; cook, stirring, until softened, about 10 minutes.
2. Add carrot and bell pepper; cook, stirring constantly, until crisp-tender, about 5 minutes. Add broccoli, cauliflower, 1 cup broth, and mushrooms to skillet. Cook, stirring occasionally, until vegetables are tender, about 10 minutes.
3. In a small bowl, combine yogurt, remaining broth, mustard and black pepper. Mix well. Add yogurt mixture to vegetable mixture. Cook, stirring constantly, until mixture is heated through and thickened slightly, about 5 minutes.