Preheat oven to 425°F (220°C). On a large baking sheet, toss potatoes, carrots, parsnip, and red onion with 2 tbsp olive oil, 1 tbsp chopped rosemary, 1 tsp kosher salt, and 1/2 tsp black pepper until evenly coated and in a single layer.

Roast

Roast in the preheated oven for 25–30 minutes, turning once halfway through, until vegetables are golden at the edges and tender when pierced with a fork. Keep warm while you cook the chicken.

Chicken

Season chicken

Pat chicken thighs dry with paper towels. In a small bowl combine 1.5 tsp kosher salt, 1 tsp black pepper, and 1/2 tsp smoked paprika. Rub the spice mix evenly over both sides of each thigh and let rest at room temperature for 10 minutes.

Sear

Heat a large oven-safe skillet over medium-high heat. Add 2 tbsp olive oil. When shimmering, place chicken thighs skin-side down, pressing gently for even contact. Sear without moving 6–8 minutes until skin is deep golden and crisp.

Finish in oven

Flip thighs skin-side up, add 1 tbsp butter to the pan and transfer skillet to the oven (with the vegetables still roasting or after they finish). Roast at 425°F (220°C) for 8–10 minutes until an instant-read thermometer inserted into the thickest part reads 165°F (74°C). Remove skillet and transfer chicken to a plate; tent with foil and let rest 5 minutes.

Herb Butter & Assembly

Make herb butter

While chicken rests, combine 2 tbsp softened unsalted butter, 2 tbsp chopped parsley, 1 tsp lemon juice, lemon zest from the reserved lemon, 2 tbsp fresh thyme leaves (or 2 tsp chopped if dry), and 1/4 tsp flaky sea salt. Mix until smooth.

Pan sauce (optional)

Place the skillet used for chicken back on medium heat. Add minced garlic (3 cloves) and sauté 30 seconds until fragrant. Add remaining 1 tbsp butter and the lemon juice from the lemon; scrape browned bits from the pan with a wooden spoon and simmer 30–45 seconds. If the pan is very dry, add 1–2 tbsp water or chicken stock to deglaze. Turn off heat.

Plate and finish

Divide roasted root vegetables among 4 plates. Place a rested chicken thigh on each. Spoon pan sauce over the chicken and top each thigh with a generous dollop (about 1/2 tbsp) of herb butter so it melts over the warm meat. Sprinkle with additional chopped parsley and flaky sea salt to finish.