Preheat the oven. Heat to 425°F (220°C).
Place a large rimmed sheet pan inside to warm up. A hot pan helps achieve crisp edges.
Prep the marinade. In a large bowl, whisk olive oil, lemon zest, lemon juice, garlic, Italian seasoning, smoked paprika, red pepper flakes (if using), and half the salt and pepper.
Season the chicken. Pat the chicken dry with paper towels. Add it to the bowl and toss to coat.
Let it sit while you prep the vegetables, about 10 minutes. This brief rest boosts flavor.
Prep the veggies. In another bowl, toss potatoes, carrots, broccoli, onion, and bell pepper with a drizzle of olive oil and the remaining salt and pepper. Keep pieces around 1-inch so they roast evenly.
Start the sturdy vegetables first. Remove the hot sheet pan, line it with parchment (optional for easy cleanup), and spread the potatoes and carrots in a single layer.
Roast for 10 minutes to give them a head start.
Add the rest. Pull the pan out, flip the potatoes and carrots, then scatter the broccoli, onion, and bell pepper. Nestle the marinated chicken pieces among the veggies, leaving space between pieces for browning.
Roast until done. Cook for 18–22 minutes, or until the chicken reaches an internal temperature of 165°F (74°C). If you want extra color, broil for 1–2 minutes at the end, watching closely.
Finish and serve. Taste and adjust seasoning with a squeeze of lemon and a pinch of salt.
Sprinkle with chopped parsley or dill. Serve as is, or with rice, quinoa, or bread to catch the juices.