Preheat the oven: Heat to 425°F (220°C). Line a large sheet pan with parchment for easy cleanup.
Pat the chicken dry: This helps it brown.
If using breasts, pound to even thickness or cut into 2–3 pieces for even cooking.
Mix the marinade: In a small bowl, combine olive oil, lemon juice, garlic, smoked paprika, oregano, thyme, red pepper flakes, 1 teaspoon salt, and 1/2 teaspoon black pepper.
Season the chicken: Toss the chicken with about half of the marinade. Let it sit while you prep the veggies (5–10 minutes is enough).
Prep the veggies: Add broccoli, cauliflower, zucchini, bell pepper, and red onion to a large bowl. Pour over the remaining marinade and toss to coat.
Add another pinch of salt if needed.
Arrange on the pan: Spread veggies in a single layer. Nestle the chicken pieces among them, leaving a little space so everything roasts, not steams.
Roast: Bake for 20–25 minutes, until the chicken hits an internal temp of 165°F (74°C) and the veggies are tender with browned edges. Thighs may need the full 25 minutes.
Optional finish: Dot the hot pan with butter and a squeeze of lemon.
Toss gently so the butter melts into a glossy, flavorful coating.
Garnish and cool: Sprinkle with chopped parsley. Let cool for 10–15 minutes before portioning into meal-prep containers.
Portion: Divide the chicken and veggies into 4–5 containers. If using avocado or Parmesan, add those right before eating, not during storage.