chicken and dumplings

INGREDIENTS

• 5 pounds bone-in, skin-on chicken thighs
• Table salt and ground black pepper
• 4 teaspoons vegetable oil
• 4 tablespoons unsalted butter (1/2 stick)
• 2 medium leeks , white and llight green parts only, cut in half lengthwise and then into 1-inch pieces
• 1 large onion, minced
• 6 tablespoons unbleached all-purpose flour
• 1/4 cup dry sherry
• 4 1/2 cups low-sodium chicken broth
• 1/4 cup whole milk
• 1 teaspoon minced fresh thyme leaves
• 2 bay leaves
• 1 cup frozen green peas
• 3 tablespoons minced fresh tarragon leaves
• 2 cups unbleached all-purpose flour
• 1 tablespoon baking powder
• 1 teaspoon table salt
• 1 cup whole milk
• 3 tablespoons reserved chicken fat (or unsalted butter)