Turkey Shepherds Pie

Name: Turkey Shepherd’s Pie
Ingredients: 1 medium yellow onion; 2 medium carrots; 2 medium stalks celery; 12 sprigs fresh thyme, or 3/4 teaspoon dried thyme; 5 leaves fresh sage, or 1/2 teaspoon dried sage; 1 pound boneless, skinless cooked turkey (about 3 cups); 2 tablespoons unsalted butter; 1/2 cup frozen peas; 1 teaspoon kosher salt, plus more as needed; 1/2 teaspoon freshly ground black pepper, plus more as needed; 2 cups cold gravy; 3 cups mashed potatoes; 1 large egg yolk