Preheat oven to 425°F (220°C). Toss potatoes, carrots, parsnip, half the olive oil, half the garlic, salt, and pepper on a rimmed baking sheet.
Pat chicken dry and rub with remaining oil, herbs, lemon zest, and remaining garlic. Season again lightly.
Nestle thighs skin-side up among the vegetables. Drizzle lemon juice over everything and scatter red pepper flakes if using.
Roast 35–45 minutes until vegetables are tender and chicken reaches 165°F, broiling 2–3 minutes at the end if you want extra-crisp skin.
Let rest 5 minutes before serving. Spoon pan juices over chicken.