Preheat: Heat the oven to 400°F (200°C) and line a large baking tray.
Initial Toss: Toss the potatoes and carrots in half of the garlic-herb marinade.
First Roast: Roast the potatoes and carrots for 15–20 minutes to ensure they cook through.
Second Toss: Toss the chicken chunks, broccoli florets, and onion wedges in the remaining marinade.
Assemble: Add the chicken and remaining vegetables to the pan, arranging everything in a single layer.
Final Roast: Roast for another 20 minutes (roughly 25–30 minutes total for the chicken) until the chicken reaches an internal temperature of 165°F and vegetables are tender.
Garnish: Finish the dish with fresh parsley and a squeeze of lemon juice before serving.