Searing the chicken on the stovetop and then simmering it in the oven gets you crisp skin and tender meat. Add bright citrus and salty olives to the rich thighs, and you’ve got a perfectly balanced dish.
Preheat oven to 375°F. Heat oil in a deep 10-inch ovenproof skillet over medium. Sprinkle chicken evenly with salt. Arrange chicken, skin side down, in skillet. Cook, undisturbed, until chicken skin is browned and crisp, about 15 minutes.
Meanwhile, juice 2 oranges into a bowl. Cut remaining orange into 8 wedges.
Transfer chicken, skin side up, to a plate; do not wipe skillet clean. Add leek to skillet; cook over medium, stirring and scraping bottom of skillet with a wooden spoon to loosen browned bits, just until leek begins to soften, about 4 minutes. Add garlic, ginger, and crushed red pepper, and cook, stirring constantly, until very fragrant, about 2 minutes. Stir in lemon wedges, reserved orange wedges, thyme sprigs, and olives.
Nestle chicken, skin side up, in skillet, making sure all chicken is touching bottom of skillet. Pour reserved orange juice into skillet around chicken; bring to a simmer over medium-high. Carefully transfer skillet to oven, and roast until chicken is cooked through and leek is very tender, about 40 minutes. Remove and discard thyme sprigs. Garnish with thyme leaves.