Preheat oven to 400°F. Spray baking sheet with nonstick cooking spray.
Mix mustard, honey, oil, salt and pepper in large bowl. Add Brussels sprouts, sweet potatoes and onions, tossing to coat. Spread vegetables on baking sheet, leaving space in middle.
Add chicken to bowl and toss with remaining sauce. Place chicken in middle of baking sheet.
Bake 30 minutes, or until chicken is cooked through and vegetables are tender.