chicken and dumplings

Name: chicken and dumplings
Cuisine: European
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)