Preheat and prep the pan: Heat your oven to 425°F (220°C). Line a large sheet pan with parchment paper or lightly oil it to prevent sticking.
Make the marinade: In a bowl, whisk 2 tablespoons olive oil, lemon juice, garlic, smoked paprika, oregano, thyme, cumin, red pepper flakes (if using), 1 teaspoon salt, and 1/4 teaspoon pepper.
Season the chicken: Add chicken pieces to the marinade and toss to coat.
Let sit while you prep the vegetables (5–10 minutes is enough, but up to 30 minutes is great).
Toss the veggies: In a large bowl, combine potatoes, broccoli, cauliflower, bell pepper, onion, and zucchini. Drizzle with remaining 1 tablespoon olive oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Toss to coat.
Stagger the roasting: Spread the potatoes, broccoli, and cauliflower on the sheet pan first.
These take a bit longer. Roast for 10 minutes.
Add the rest: Remove the pan, add bell pepper, onion, zucchini, and the marinated chicken. Spread everything into an even layer with space between pieces for better browning.
Roast to finish: Return to the oven and roast 18–22 minutes, until the chicken is cooked through (165°F/74°C internal temperature) and the vegetables are tender with golden edges.
Optional broil: For extra color, broil for 2–3 minutes at the end.
Watch closely to avoid burning.
Garnish and serve: Squeeze a little extra lemon over the top and sprinkle with chopped parsley. Serve as is, or with rice, quinoa, or warm pita.