Preheat and prep: Position an oven rack in the middle and preheat the oven to 425°F (220°C). High, even heat is important here — it promotes Maillard browning on the chicken skin and caramelization on the Brussels sprouts without overcooking the interior.

Prepare the chicken: Pat the thighs very dry with paper towels (moisture prevents crisping). In a small bowl combine 1 tbsp olive oil, 1 tsp kosher salt, 1/2 tsp black pepper, the chopped rosemary and thyme (reserve a pinch of herbs for finishing) and the smoked paprika if using. Rub this mixture evenly under and over the chicken skin, massaging it into any crevices. Let sit at room temperature while you prep the veg (10–15 minutes). Why: rubbing oil and herbs under the skin flavors the meat directly and drying the skin is the single best step for crisp skin.

Prep the sweet potatoes: Peel (or scrub well if you prefer skin-on) and cut the sweet potatoes into 1-inch cubes for even cooking. Place them in a medium pot and cover with cold water by 1 inch. Add 1/2 tsp kosher salt to the water. Starting in cold water lets them heat uniformly so they cook through at the same time. Bring to a simmer, then cook until easily pierced with a fork, about 15–18 minutes.

Prep the Brussels sprouts: While the potatoes simmer, trim the stem ends of the Brussels sprouts and halve any large ones so pieces are similar in size. Toss the sprouts with the remaining 1 tbsp olive oil, the remaining 1 tsp kosher salt and 1/4 tsp black pepper. Why: halving increases surface area so more caramelization (browned, nutty flavor) occurs in the oven.

Sear the chicken: Heat a 10–12 inch ovenproof skillet (cast-iron preferred) over medium-high heat until very hot, then add the chicken thighs skin-side down (no extra oil if skillet is well-seasoned; otherwise add a thin film). Sear without moving for 5–7 minutes until the skin is deep golden-brown and releases easily from the pan. Why: initial high-heat sear renders fat and creates a crisp, flavorful crust; it also builds fond (browned bits) that enrich the pan juices.

Roast the chicken and sprouts together: After searing, flip the chicken so skin-side is up. Arrange the prepared Brussels sprouts cut-side down around the thighs in the same skillet or transfer everything to a rimmed sheet pan (use the skillet if you have one large enough). Roast in the preheated oven for 22–28 minutes, until the chicken reaches an internal temperature of 165°F (74°C) at the thickest part (use an instant-read thermometer) and the Brussels sprouts are deeply caramelized at the edges. Why: roasting finishes cooking the chicken gently and concentrates flavors in the sprouts; checking temperature guarantees safety without overcooking.

Finish the sweet potato mash: Drain the cooked sweet potatoes and return them to the warm pot over very low heat for 1 minute to evaporate excess moisture — this prevents watery mash. Off heat, add the 1 tbsp butter, 1/8–1/4 cup of the reserved hot cooking water (or warm water) to loosen as needed, and mash with a potato masher until mostly smooth but with a little texture. Season with 1/4 tsp salt and a few grinds of black pepper and fold in a teaspoon of the reserved fresh herbs if desired. Why: finishing in the warm pot helps evaporate water and keep the mash creamy without needing cream; adding hot liquid gradually prevents a gluey texture.

Rest the chicken: When the chicken reaches 165°F, remove from oven and tent loosely with foil for 5 minutes before serving. Resting allows juices to redistribute for juicier meat and keeps the skin from steaming as you plate. While the chicken rests, if there are pan juices, place the skillet over medium heat for 1 minute to loosen them, spoon some over the thighs to finish, and toss a few roasted herbs into the mash or sprouts for brightness.

Plate and serve: Spoon a generous portion of sweet potato mash onto each plate, add a portion of roasted Brussels sprouts, and top with a chicken thigh. Spoon pan juices over the chicken and garnish with the reserved fresh herbs. Serve hot. Why: layering the mash first gives a soft bed that contrasts with the crisp-skinned chicken and caramelized sprouts, creating a balanced plate in texture and flavor.