Make-Ahead Turkey Gravy

Ingredients (9)

• 2 turkey wings (1-1/2 to 2 pounds)
• 2 medium onions, quartered
• 2 cartons (32 ounces each) reduced-sodium chicken broth, divided
• 2 medium carrots, cut into 2-inch pieces
• 2 celery ribs with leaves, cut into 2-inch pieces
• 4 fresh thyme sprigs
• 1/2 cup plus 2 tablespoons all-purpose flour
• 1 tablespoon butter
• 1/4 teaspoon pepper