Preparation

• Have ready four individual pie plates or dishes with a 1-cup capacity. (If a little less than 1 cup, use five plates.) Position a rack in the center of the oven and heat to 400°F
• To prepare the pie shells, working with one piece of dough at a time, roll the disk into an 8-inch-diameter circle. Transfer the dough to one of the plates, gently tucking it into the plate. Roll and pinch the overhanging dough into an upstanding ridge, and crimp. Refrigerate. Repeat with the other pieces of dough.

Make the filling

• Melt the butter over medium-low heat in a medium Dutch oven. Add the onion and celery, and cook, stirring, until softened, about 3 minutes. Add the flour and cook, stirring, for about 1 minute. Whisk in the chicken broth about a third at a time. As it thickens, add the next third. Whisk in the cream, sage, thyme, 1/2 tsp. salt, and 1/4 tsp. pepper. Bring to a simmer, and cook until thickened to a gravy-like consistency, about 5 minutes, then remove from the heat. Add the cooked chicken and vegetables. Mix thoroughly and set aside to cool completely.
• Position two racks in the oven, one in the bottom third and the other in the center. Line a large rimmed baking sheet with aluminum foil. Place the pie shells on the baking sheet. Spoon enough filling (about 1 cup) into each pie shell so that it comes just below the upper edge of the dough; smooth with a spoon. Bake the pies on the lower oven rack until the crust begins to brown, about 25 minutes.

Make the topping

• Meanwhile, scoop the flesh of the sweet potatoes into a medium bowl. Melt the butter in a large saucepan over medium heat. Add the onion and cook, stirring often, until soft, about 8 minutes. Stir in the curry powder and cook, stirring, until fragrant, about 30 seconds. Add the sweet potatoes, stirring with a wooden spoon to break them up; season with 1/4 tsp. salt. Remove the pan from the heat and mash, gradually adding the milk. Add salt to taste.
• Spoon a generous mound of the sweet potatoes over each pie in an even layer. Reduce the oven temperature to 375°F. Put the sheet on the center oven rack, and continue to bake until the filling bubbles around the edge and the crust is golden, about 20 minutes. Transfer the pies to a cooling rack and cool for at least 15 minutes before serving.