Preheat oven to 400°F (200°C). Pat chicken dry and season with salt and pepper.
Heat 1 Tbsp olive oil in a large oven-safe pan. Sear chicken skin-side down 15 min until golden, then flip and cook 15–20 min more, to 165°F. Remove and set aside.
In the same pan, melt butter with 2 Tbsp olive oil over medium heat. Sauté onions and leeks until caramelized, about 15 min. Add garlic; cook 2 min.
Stir in stock and wine; simmer 15 min to reduce. Whisk in crème fraîche, lemon juice, salt, and pepper; adjust seasoning.
Nestle chicken into sauce. Add lemon slices and thyme on top. Roast uncovered 15 min, broil briefly for extra crispiness if desired.
Serve hot, spooning sauce over chicken and mashed potatoes.
