Pastry crust: In a medium bowl, combine flour, sugar and salt. Cut butter into pieces into the flour mixture, and using a pastry blender, a fork, or your hands break the cold butter into pea size pieces.
In a measuring cup, add cold water and ice. Gradually add the ice water a little at a time, starting with 5 tablespoons and if it's too dry add another tablespoon, gathering the flour into the center of the bowl. I like to use my hands, and once the dough has formed a large disc, wrap in plastic saran and refrigerate for minimum 30 minutes or overnight. Handle the dough as little as possible to prevent it from becoming tough.
Filling: In a large bowl, gently combine apples, lemon juice, sugar, and cinnamon.
Preheat oven to 375°F. Line a baking sheet with parchment paper. Remove the pastry dough from the refrigerator, lightly flour a flat surface, and roll out the dough to a 12 inch circle. Then transfer to the lined baking sheet.
Arrange the apple mixture, cranberries and brie evenly over the pastry dough leaving about 3 inches from the edges.
Fold the edges inward toward the center, creating a pleated design. Then brush with egg wash. Bake for 40 minutes until the crust is golden and the apples have softened, cranberries are cooked and brie has melted.
Allow to cool for about 15 minutes. Garnish with fresh rosemary and if so desired add chopped walnuts.