Turkey Shepherds Pie with Two Potato Topping

Name: Turkey Shepherd's Pie with Two-Potato Topping
Ingredients: 3 tablespoons unsalted butter, divided, 1 tablespoon of it melted; 1 medium onion, finely chopped; 1 medium celery rib, finely chopped; 1 medium carrot, finely chopped; 4 cups diced turkey meat; 1 cup turkey gravy; 1 tablespoon chopped flat-leaf parsley; 2 teaspoons finely chopped sage; Kosher s alt; Freshly ground black pepper; 2 cups mashed potatoes; 1 cup mashed sweet potatoes; 2 tablespoons minced chives; 1 large egg, beaten