Melt the butter in a large skillet and add the onion and apples. Cook, stirring often, for 4 - 6 minutes or until softened.
Stir in the sage and cook for an additional minute, until fragrant. Remove from heat and cool.
Once cooled, stir in the chicken, cheese, salt and pepper and stir to combine.
Preheat oven to 400º F. Line a sheet pan with parchment paper.
Thaw pastry dough according to package instructions until it's easy to roll out; not quite to room temperature.
Dust your work surface with flour and roll out the puff pastry to about 10"x12" rectangle. Cut each sheet into 4 smaller rectangles.
Divide the mixture and spoon it onto 4 of the rectangles, leaving a 3/4" border all around.
Place another rectangle over top and use a fork to crimp and seal the filling within the puff pastry pocket. Cut 3- 4 slits on the top of the pastry and brush with egg wash (egg whisked with 1 teaspoon of water).
Place the filled pastry pocket on the prepared sheet pan and bake until golden brown, about 20 minutes. Cool slightly before serving.