1. Preheat oven to 450°F.
2. In a large bowl, add Brussels sprouts, cauliflower, broccoli, onion & lemon. Drizzle 3 Tbsp oil, rosemary & pepper over vegetables. Toss to combine.
3. Lightly coat a foil-wrapped sheet pan with cooking spray. Spread vegetables evenly on pan & roast 20-25 minutes.
4. In a small bowl, combine garlic, mustard, Worcestershire sauce & 1 Tbsp oil. Coat chicken with marinade & set aside.
5. Remove pan from oven, place chicken on sheet pan & roast another 12-15, until chicken is cooked through & Brussels sprouts are tender.