Heat the oven to 400°F and arrange a rack in the middle. Coat a 13-by-9-inch baking dish with butter and set aside. Melt 2 tablespoons butter over medium heat in a large frying pan until foaming. Add onions and sage and cook until sage aroma is released, about 2 minutes. Add potato, celery, and carrots and season well with salt and freshly ground black pepper. Cook until carrots and onions are just beginning to soften, about 6 minutes. Remove mixture from heat, stir in peas, parsley, and creamed chicken, and season with salt and pepper. Turn filling into the prepared baking dish. Whisk egg together with 1 tablespoon water and a pinch of salt until evenly mixed. Set aside. With kitchen shears, cut dough to fit over the baking dish. Place dough over filling and tuck into the edges of the dish. Brush dough with reserved egg wash and cut 10 evenly distributed slits in the top to vent. Cook until crust is golden brown and mixture is bubbling, about 30 to 35 minutes. Let sit 10 minutes before serving.