Bring ingredients to room temperature (10–15 minutes). Pat the chicken thighs very dry with paper towels—dry skin browns and crisps better. While the chicken rests, peel and cut the sweet potato into 1-inch cubes so it cooks evenly. Bring a medium pot of salted water to a boil for the potatoes. (Why: room-temperature meat cooks more evenly; dryness helps a good sear.)

Season the chicken: in a small bowl combine 3/4 tsp kosher salt (reserve 1/4 tsp for the potatoes), 1/2 tsp black pepper, chopped rosemary, chopped thyme and the minced garlic. Drizzle 1 tbsp olive oil over the thighs and rub the herb-garlic mixture under the skin where possible and on the skin. (Why: rubbing under the skin infuses the meat with herb flavor; oil promotes even browning.)

Start the sweet potato: when the water is boiling, add the cubed sweet potato and simmer gently until fork-tender, about 12–15 minutes depending on cube size. Salt the water with the reserved 1/4 tsp salt. Drain well in a colander and return to the hot pot off the heat. (Why: simmering tenderizes the sweet potato for a smooth mash; salting the cooking water seasons the interior.)

Preheat the oven to 425°F (220°C). While it heats, place a 10–12 inch ovenproof skillet (cast iron is ideal) over medium-high heat. Add the remaining 1/2 tbsp olive oil and let it warm until shimmering but not smoking. (Why: a hot pan gives an immediate sear that crisps the skin and renders fat.)

Sear the chicken thighs skin-side down in the hot skillet for 3–4 minutes without moving them. You’re aiming for a deep golden-brown, not burnt; press gently with a spatula so the skin makes even contact. Flip and sear the other side 1 minute just to color. (Why: searing develops Maillard flavors and starts rendering fat, which leads to crisp skin and savory depth.)

Transfer the skillet to the preheated oven and roast for 18–22 minutes, until an instant-read thermometer inserted into the thickest part (avoiding bone) reads 165°F (74°C). If the skin isn’t quite crisp after cooking, use the broiler 1–2 minutes while watching closely. (Why: finishing in the oven cooks through gently and consistently; temperature ensures safety while avoiding overcooking.)

While the chicken roasts, finish the mash: to the drained sweet potatoes add butter, warmed milk, a pinch of salt and a generous grind of pepper and 1/2 tsp of chopped thyme. Mash to your desired texture — for extra silkiness, push the potatoes through a ricer or use a handheld masher. Taste and adjust seasoning. (Why: warm fat and milk emulsify more easily into the potatoes for a silky mouthfeel; fresh herbs lift the sweetness.)

Reheat the steamed green beans in a skillet over medium heat with a drizzle of olive oil and a pinch of salt for 2–3 minutes until just warmed through; toss with a squeeze of lemon if using. (Why: briefly reheating maintains a bright color, fresh snap and removes any chilled wateriness.)

Rest the chicken on a cutting board for 5 minutes before serving. Resting allows juices to redistribute so the meat stays moist when cut. Spoon a bed of sweet potato mash onto each plate, place a roasted thigh on top (or halved if you prefer), arrange the green beans to the side and finish with a light drizzle of pan juices from the skillet and a little lemon zest over the mash. (Why: plating while the thigh rests makes for a hot, juicy presentation and a glossy finish.)

Final seasoning: taste and finish with a final pinch of flaky salt over the chicken skin (if you have it), extra cracked pepper, and the reserved chopped rosemary or thyme for a fragrant, professional finish. Serve immediately. (Why: finishing salts and fresh herbs amplify aroma and deliver the last layer of flavor.)