Chicken and Biscuits

Name: Chicken and Biscuits
Ingredients: 2 tablespoons vegetable oil; 1 1/2 pounds skin-on, bone-in chicken thighs; Kosher salt, freshly ground pepper; 8 medium carrots (about 1 lb.), peeled, cut into 2 pieces; 1 small celery root (about 12 ounces), peeled, cut into 1/2 pieces; 1 medium onion, chopped; 1 leek, white and pale-green parts only, chopped; 1/4 cup (1/2 stick) unsalted butter; 1/4 cup all-purpose flour; 1 cup dry white wine; 4 cups low-sodium chicken broth; 4 sprigs thyme; 1 sprig rosemary; 2 bay leaves; 1 1/2 teaspoons (or more) Sherry vinegar; 2 1/4 cups all-purpose flour plus more; 2 teaspoons kosher salt; 3/4 teaspoon baking powder; 1/2 cup (1 stick) cold unsalted butter, cut into 1/2 cubes; 3/4 cup buttermilk; 2 tablespoons crème fraîche or sour cream; 2 tablespoons chopped fresh chives; 1 large egg, beaten to blend