Pat the chicken pieces dry with paper towels, trim any fat, and cut into 1/2-inch pieces; place in a medium bowl.
In a small bowl, whisk together parsley, oregano, garlic powder, onion powder, paprika, seasoned salt, pepper, and optional red pepper flakes.
Toss half the seasoning mixture with the chicken, 2 tablespoons olive oil, and the lemon zest and juice until well coated; cover and refrigerate up to 30 minutes.
Preheat the oven to 425°F (220°C) and line a large sheet pan with parchment paper.
Halve the baby carrots lengthwise, halve or quarter the baby red potatoes as needed, and coarsely chop the onion.
Place carrots, potatoes, and onion on the prepared sheet pan; drizzle with 2 tablespoons olive oil and most of the remaining seasoning (reserve about 1 teaspoon), then toss to coat. Roast for 20 minutes.
While the first roast is happening, chop the broccoli into bite-sized pieces.
Remove the pan, toss the roasted vegetables, move them to one side of the pan, add broccoli and minced garlic to the other side, drizzle with the last tablespoon olive oil and sprinkle the reserved seasoning; toss to coat and combine the vegetables on the sheet as desired.
Drain any excess liquid from the marinated chicken, spread the chicken pieces on the pan so they are in a single layer and not touching, then return the pan to the oven.
Roast for 15–20 minutes more, until the chicken reaches 165°F (74°C) and vegetables are tender. If desired, broil on high for 1–2 minutes to add char—watch closely to avoid burning.
Remove from oven, sprinkle with freshly grated Parmesan, and serve immediately or portion with cooked quinoa or rice for meal prep.
To meal prep, cook 1 cup uncooked quinoa or rice according to package directions and divide evenly among four containers, then add equal portions of the sheet-pan chicken and vegetables.
