For this recipe, the filling thickens as it bakes. It is very important to leave space for steam to escape from the pies - that moisture is from the fruit and as the steam escapes, the filling will reduce and thicken. You should also let the pies cool completely. This will allow the filling to thicken and set. Some recipes call for cornstarch to thicken pie filling, but for this one, I make it without cornstarch.