Chicken and Biscuits

INGREDIENTS

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